diff --git a/lib/url.c b/lib/url.c
index de43df4f7b..adef2cdb36 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -136,15 +136,6 @@ bool curl_win32_idn_to_ascii(const char *in, char **out);
 #include "curl_memory.h"
 #include "memdebug.h"
 
-/* Count of the backend ssl objects to allocate */
-#ifdef USE_SSL
-#  ifndef CURL_DISABLE_PROXY
-#    define SSL_BACKEND_CNT 4
-#  else
-#    define SSL_BACKEND_CNT 2
-#  endif
-#endif
-
 static void conn_free(struct connectdata *conn);
 
 /* Some parts of the code (e.g. chunked encoding) assume this buffer has at
@@ -752,7 +743,9 @@ static void conn_shutdown(struct Curl_easy *data, struct connectdata *conn)
   /* close the SSL stuff before we close any sockets since they will/may
      write to the sockets */
   Curl_ssl_close(data, conn, FIRSTSOCKET);
+#ifndef CURL_DISABLE_FTP
   Curl_ssl_close(data, conn, SECONDARYSOCKET);
+#endif
 
   /* close possibly still open sockets */
   if(CURL_SOCKET_BAD != conn->sock[SECONDARYSOCKET])
@@ -1667,18 +1660,35 @@ static struct connectdata *allocate_conn(struct Curl_easy *data)
      Note that these backend pointers can be swapped by vtls (eg ssl backend
      data becomes proxy backend data). */
   {
-    size_t sslsize = Curl_ssl->sizeof_ssl_backend_data;
-    char *ssl = calloc(SSL_BACKEND_CNT, sslsize);
+    size_t onesize = Curl_ssl->sizeof_ssl_backend_data;
+    size_t totalsize = onesize;
+    char *ssl;
+
+#ifndef CURL_DISABLE_FTP
+    totalsize *= 2;
+#endif
+#ifndef CURL_DISABLE_PROXY
+    totalsize *= 2;
+#endif
+
+    ssl = calloc(1, totalsize);
     if(!ssl) {
       free(conn);
       return NULL;
     }
     conn->ssl_extra = ssl;
-    conn->ssl[0].backend = (void *)ssl;
-    conn->ssl[1].backend = (void *)(ssl + sslsize);
+    conn->ssl[FIRSTSOCKET].backend = (void *)ssl;
+#ifndef CURL_DISABLE_FTP
+    ssl += onesize;
+    conn->ssl[SECONDARYSOCKET].backend = (void *)ssl;
+#endif
 #ifndef CURL_DISABLE_PROXY
-    conn->proxy_ssl[0].backend = (void *)(ssl + 2 * sslsize);
-    conn->proxy_ssl[1].backend = (void *)(ssl + 3 * sslsize);
+    ssl += onesize;
+    conn->proxy_ssl[FIRSTSOCKET].backend = (void *)ssl;
+#ifndef CURL_DISABLE_FTP
+    ssl += onesize;
+    conn->proxy_ssl[SECONDARYSOCKET].backend = (void *)ssl;
+#endif
 #endif
   }
 #endif
diff --git a/lib/vtls/bearssl.c b/lib/vtls/bearssl.c
index a4d2d910b6..77e22cf3e0 100644
--- a/lib/vtls/bearssl.c
+++ b/lib/vtls/bearssl.c
@@ -373,6 +373,8 @@ static CURLcode bearssl_connect_step1(struct Curl_easy *data,
   struct in_addr addr;
 #endif
 
+  DEBUGASSERT(backend);
+
   switch(SSL_CONN_CONFIG(version)) {
   case CURL_SSLVERSION_SSLv2:
     failf(data, "BearSSL does not support SSLv2");
@@ -530,6 +532,8 @@ static CURLcode bearssl_run_until(struct Curl_easy *data,
   ssize_t ret;
   int err;
 
+  DEBUGASSERT(backend);
+
   for(;;) {
     state = br_ssl_engine_current_state(&backend->ctx.eng);
     if(state & BR_SSL_CLOSED) {
@@ -602,6 +606,8 @@ static CURLcode bearssl_connect_step2(struct Curl_easy *data,
   struct ssl_backend_data *backend = connssl->backend;
   CURLcode ret;
 
+  DEBUGASSERT(backend);
+
   ret = bearssl_run_until(data, conn, sockindex,
                           BR_SSL_SENDAPP | BR_SSL_RECVAPP);
   if(ret == CURLE_AGAIN)
@@ -624,6 +630,7 @@ static CURLcode bearssl_connect_step3(struct Curl_easy *data,
   CURLcode ret;
 
   DEBUGASSERT(ssl_connect_3 == connssl->connecting_state);
+  DEBUGASSERT(backend);
 
   if(conn->bits.tls_enable_alpn) {
     const char *protocol;
@@ -689,6 +696,8 @@ static ssize_t bearssl_send(struct Curl_easy *data, int sockindex,
   unsigned char *app;
   size_t applen;
 
+  DEBUGASSERT(backend);
+
   for(;;) {
     *err = bearssl_run_until(data, conn, sockindex, BR_SSL_SENDAPP);
     if (*err != CURLE_OK)
@@ -722,6 +731,8 @@ static ssize_t bearssl_recv(struct Curl_easy *data, int sockindex,
   unsigned char *app;
   size_t applen;
 
+  DEBUGASSERT(backend);
+
   *err = bearssl_run_until(data, conn, sockindex, BR_SSL_RECVAPP);
   if(*err != CURLE_OK)
     return -1;
@@ -847,6 +858,7 @@ static bool bearssl_data_pending(const struct connectdata *conn,
 {
   const struct ssl_connect_data *connssl = &conn->ssl[connindex];
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
   return br_ssl_engine_current_state(&backend->ctx.eng) & BR_SSL_RECVAPP;
 }
 
@@ -896,6 +908,7 @@ static void *bearssl_get_internals(struct ssl_connect_data *connssl,
                                    CURLINFO info UNUSED_PARAM)
 {
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
   return &backend->ctx;
 }
 
@@ -906,6 +919,8 @@ static void bearssl_close(struct Curl_easy *data,
   struct ssl_backend_data *backend = connssl->backend;
   size_t i;
 
+  DEBUGASSERT(backend);
+
   if(backend->active) {
     br_ssl_engine_close(&backend->ctx.eng);
     (void)bearssl_run_until(data, conn, sockindex, BR_SSL_CLOSED);
diff --git a/lib/vtls/gskit.c b/lib/vtls/gskit.c
index 00449efd1d..56d48497db 100644
--- a/lib/vtls/gskit.c
+++ b/lib/vtls/gskit.c
@@ -514,6 +514,8 @@ static void cancel_async_handshake(struct connectdata *conn, int sockindex)
   struct ssl_connect_data *connssl = &conn->ssl[sockindex];
   Qso_OverlappedIO_t cstat;
 
+  DEBUGASSERT(BACKEND);
+
   if(QsoCancelOperation(conn->sock[sockindex], 0) > 0)
     QsoWaitForIOCompletion(BACKEND->iocport, &cstat, (struct timeval *) NULL);
 }
@@ -521,6 +523,7 @@ static void cancel_async_handshake(struct connectdata *conn, int sockindex)
 
 static void close_async_handshake(struct ssl_connect_data *connssl)
 {
+  DEBUGASSERT(BACKEND);
   QsoDestroyIOCompletionPort(BACKEND->iocport);
   BACKEND->iocport = -1;
 }
@@ -538,6 +541,9 @@ static int pipe_ssloverssl(struct connectdata *conn, int sockindex,
   int ret = 0;
   char buf[CURL_MAX_WRITE_SIZE];
 
+  DEBUGASSERT(BACKEND);
+  DEBUGASSERT(connproxyssl->backend);
+
   if(!connssl->use || !connproxyssl->use)
     return 0;   /* No SSL over SSL: OK. */
 
@@ -602,6 +608,7 @@ static int pipe_ssloverssl(struct connectdata *conn, int sockindex,
 static void close_one(struct ssl_connect_data *connssl, struct Curl_easy *data,
                       struct connectdata *conn, int sockindex)
 {
+  DEBUGASSERT(BACKEND);
   if(BACKEND->handle) {
     gskit_status(data, gsk_secure_soc_close(&BACKEND->handle),
               "gsk_secure_soc_close()", 0);
@@ -633,6 +640,8 @@ static ssize_t gskit_send(struct Curl_easy *data, int sockindex,
   CURLcode cc = CURLE_SEND_ERROR;
   int written;
 
+  DEBUGASSERT(BACKEND);
+
   if(pipe_ssloverssl(conn, sockindex, SOS_WRITE) >= 0) {
     cc = gskit_status(data,
                       gsk_secure_soc_write(BACKEND->handle,
@@ -658,6 +667,8 @@ static ssize_t gskit_recv(struct Curl_easy *data, int num, char *buf,
   int nread;
   CURLcode cc = CURLE_RECV_ERROR;
 
+  DEBUGASSERT(BACKEND);
+
   if(pipe_ssloverssl(conn, num, SOS_READ) >= 0) {
     int buffsize = buffersize > (size_t) INT_MAX? INT_MAX: (int) buffersize;
     cc = gskit_status(data, gsk_secure_soc_read(BACKEND->handle,
@@ -731,6 +742,7 @@ static CURLcode gskit_connect_step1(struct Curl_easy *data,
 #endif
 
   /* Create SSL environment, start (preferably asynchronous) handshake. */
+  DEBUGASSERT(BACKEND);
 
   BACKEND->handle = (gsk_handle) NULL;
   BACKEND->iocport = -1;
@@ -960,6 +972,7 @@ static CURLcode gskit_connect_step2(struct Curl_easy *data,
   CURLcode result;
 
   /* Poll or wait for end of SSL asynchronous handshake. */
+  DEBUGASSERT(BACKEND);
 
   for(;;) {
     timediff_t timeout_ms = nonblocking? 0: Curl_timeleft(data, NULL, TRUE);
@@ -1016,6 +1029,7 @@ static CURLcode gskit_connect_step3(struct Curl_easy *data,
   CURLcode result;
 
   /* SSL handshake done: gather certificate info and verify host. */
+  DEBUGASSERT(BACKEND);
 
   if(gskit_status(data, gsk_attribute_get_cert_info(BACKEND->handle,
                                                     GSK_PARTNER_CERT_INFO,
@@ -1208,6 +1222,8 @@ static int gskit_shutdown(struct Curl_easy *data,
   char buf[120];
   int loop = 10; /* don't get stuck */
 
+  DEBUGASSERT(BACKEND);
+
   if(!BACKEND->handle)
     return 0;
 
@@ -1271,6 +1287,7 @@ static int gskit_check_cxn(struct connectdata *cxn)
   int errlen;
 
   /* The only thing that can be tested here is at the socket level. */
+  DEBUGASSERT(BACKEND);
 
   if(!BACKEND->handle)
     return 0; /* connection has been closed */
@@ -1290,6 +1307,7 @@ static void *gskit_get_internals(struct ssl_connect_data *connssl,
                                  CURLINFO info UNUSED_PARAM)
 {
   (void)info;
+  DEBUGASSERT(BACKEND);
   return BACKEND->handle;
 }
 
diff --git a/lib/vtls/gtls.c b/lib/vtls/gtls.c
index f9ef4d12b4..57493767e5 100644
--- a/lib/vtls/gtls.c
+++ b/lib/vtls/gtls.c
@@ -202,9 +202,12 @@ static CURLcode handshake(struct Curl_easy *data,
 {
   struct ssl_connect_data *connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *backend = connssl->backend;
-  gnutls_session_t session = backend->session;
+  gnutls_session_t session;
   curl_socket_t sockfd = conn->sock[sockindex];
 
+  DEBUGASSERT(backend);
+  session = backend->session;
+
   for(;;) {
     timediff_t timeout_ms;
     int rc;
@@ -406,6 +409,8 @@ gtls_connect_step1(struct Curl_easy *data,
   const char *tls13support;
   CURLcode result;
 
+  DEBUGASSERT(backend);
+
   if(connssl->state == ssl_connection_complete)
     /* to make us tolerant against being called more than once for the
        same connection */
@@ -701,7 +706,10 @@ gtls_connect_step1(struct Curl_easy *data,
 
 #ifndef CURL_DISABLE_PROXY
   if(conn->proxy_ssl[sockindex].use) {
-    transport_ptr = conn->proxy_ssl[sockindex].backend->session;
+    struct ssl_backend_data *proxy_backend;
+    proxy_backend = conn->proxy_ssl[sockindex].backend;
+    DEBUGASSERT(proxy_backend);
+    transport_ptr = proxy_backend->session;
     gnutls_transport_push = gtls_push_ssl;
     gnutls_transport_pull = gtls_pull_ssl;
   }
@@ -1356,7 +1364,9 @@ gtls_connect_common(struct Curl_easy *data,
   /* Finish connecting once the handshake is done */
   if(ssl_connect_1 == connssl->connecting_state) {
     struct ssl_backend_data *backend = connssl->backend;
-    gnutls_session_t session = backend->session;
+    gnutls_session_t session;
+    DEBUGASSERT(backend);
+    session = backend->session;
     rc = Curl_gtls_verifyserver(data, conn, session, sockindex);
     if(rc)
       return rc;
@@ -1397,6 +1407,9 @@ static bool gtls_data_pending(const struct connectdata *conn,
   const struct ssl_connect_data *connssl = &conn->ssl[connindex];
   bool res = FALSE;
   struct ssl_backend_data *backend = connssl->backend;
+
+  DEBUGASSERT(backend);
+
   if(backend->session &&
      0 != gnutls_record_check_pending(backend->session))
     res = TRUE;
@@ -1404,6 +1417,7 @@ static bool gtls_data_pending(const struct connectdata *conn,
 #ifndef CURL_DISABLE_PROXY
   connssl = &conn->proxy_ssl[connindex];
   backend = connssl->backend;
+  DEBUGASSERT(backend);
   if(backend->session &&
      0 != gnutls_record_check_pending(backend->session))
     res = TRUE;
@@ -1421,7 +1435,10 @@ static ssize_t gtls_send(struct Curl_easy *data,
   struct connectdata *conn = data->conn;
   struct ssl_connect_data *connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *backend = connssl->backend;
-  ssize_t rc = gnutls_record_send(backend->session, mem, len);
+  ssize_t rc;
+
+  DEBUGASSERT(backend);
+  rc = gnutls_record_send(backend->session, mem, len);
 
   if(rc < 0) {
     *curlcode = (rc == GNUTLS_E_AGAIN)
@@ -1437,6 +1454,8 @@ static ssize_t gtls_send(struct Curl_easy *data,
 static void close_one(struct ssl_connect_data *connssl)
 {
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
+
   if(backend->session) {
     char buf[32];
     /* Maybe the server has already sent a close notify alert.
@@ -1479,6 +1498,8 @@ static int gtls_shutdown(struct Curl_easy *data, struct connectdata *conn,
   struct ssl_backend_data *backend = connssl->backend;
   int retval = 0;
 
+  DEBUGASSERT(backend);
+
 #ifndef CURL_DISABLE_FTP
   /* This has only been tested on the proftpd server, and the mod_tls code
      sends a close notify alert without waiting for a close notify alert in
@@ -1557,6 +1578,8 @@ static ssize_t gtls_recv(struct Curl_easy *data, /* connection data */
   struct ssl_backend_data *backend = connssl->backend;
   ssize_t ret;
 
+  DEBUGASSERT(backend);
+
   ret = gnutls_record_recv(backend->session, buf, buffersize);
   if((ret == GNUTLS_E_AGAIN) || (ret == GNUTLS_E_INTERRUPTED)) {
     *curlcode = CURLE_AGAIN;
@@ -1628,6 +1651,7 @@ static void *gtls_get_internals(struct ssl_connect_data *connssl,
 {
   struct ssl_backend_data *backend = connssl->backend;
   (void)info;
+  DEBUGASSERT(backend);
   return backend->session;
 }
 
diff --git a/lib/vtls/mbedtls.c b/lib/vtls/mbedtls.c
index d79be3d9be..b9fd26acca 100644
--- a/lib/vtls/mbedtls.c
+++ b/lib/vtls/mbedtls.c
@@ -230,6 +230,8 @@ set_ssl_version_min_max(struct Curl_easy *data, struct connectdata *conn,
   long ssl_version_max = SSL_CONN_CONFIG(version_max);
   CURLcode result = CURLE_OK;
 
+  DEBUGASSERT(backend);
+
   switch(ssl_version) {
     case CURL_SSLVERSION_DEFAULT:
     case CURL_SSLVERSION_TLSv1:
@@ -285,6 +287,8 @@ mbed_connect_step1(struct Curl_easy *data, struct connectdata *conn,
   int ret = -1;
   char errorbuf[128];
 
+  DEBUGASSERT(backend);
+
   if((SSL_CONN_CONFIG(version) == CURL_SSLVERSION_SSLv2) ||
      (SSL_CONN_CONFIG(version) == CURL_SSLVERSION_SSLv3)) {
     failf(data, "Not supported SSL version");
@@ -665,6 +669,8 @@ mbed_connect_step2(struct Curl_easy *data, struct connectdata *conn,
   const mbedtls_x509_crt *peercert;
   const char * const pinnedpubkey = SSL_PINNED_PUB_KEY();
 
+  DEBUGASSERT(backend);
+
   conn->recv[sockindex] = mbed_recv;
   conn->send[sockindex] = mbed_send;
 
@@ -844,6 +850,7 @@ mbed_connect_step3(struct Curl_easy *data, struct connectdata *conn,
   struct ssl_backend_data *backend = connssl->backend;
 
   DEBUGASSERT(ssl_connect_3 == connssl->connecting_state);
+  DEBUGASSERT(backend);
 
   if(SSL_SET_OPTION(primary.sessionid)) {
     int ret;
@@ -900,6 +907,8 @@ static ssize_t mbed_send(struct Curl_easy *data, int sockindex,
   struct ssl_backend_data *backend = connssl->backend;
   int ret = -1;
 
+  DEBUGASSERT(backend);
+
   ret = mbedtls_ssl_write(&backend->ssl, (unsigned char *)mem, len);
 
   if(ret < 0) {
@@ -924,6 +933,8 @@ static void mbedtls_close(struct Curl_easy *data,
   char buf[32];
   (void) data;
 
+  DEBUGASSERT(backend);
+
   /* Maybe the server has already sent a close notify alert.
      Read it to avoid an RST on the TCP connection. */
   (void)mbedtls_ssl_read(&backend->ssl, (unsigned char *)buf, sizeof(buf));
@@ -952,6 +963,8 @@ static ssize_t mbed_recv(struct Curl_easy *data, int num,
   int ret = -1;
   ssize_t len = -1;
 
+  DEBUGASSERT(backend);
+
   ret = mbedtls_ssl_read(&backend->ssl, (unsigned char *)buf,
                          buffersize);
 
@@ -1186,6 +1199,7 @@ static bool mbedtls_data_pending(const struct connectdata *conn,
 {
   const struct ssl_connect_data *connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
   return mbedtls_ssl_get_bytes_avail(&backend->ssl) != 0;
 }
 
@@ -1215,6 +1229,7 @@ static void *mbedtls_get_internals(struct ssl_connect_data *connssl,
 {
   struct ssl_backend_data *backend = connssl->backend;
   (void)info;
+  DEBUGASSERT(backend);
   return &backend->ssl;
 }
 
diff --git a/lib/vtls/nss.c b/lib/vtls/nss.c
index c3f40f2b96..558e3bed39 100644
--- a/lib/vtls/nss.c
+++ b/lib/vtls/nss.c
@@ -488,6 +488,9 @@ static CURLcode nss_create_object(struct ssl_connect_data *connssl,
   const int slot_id = (cacert) ? 0 : 1;
   char *slot_name = aprintf("PEM Token #%d", slot_id);
   struct ssl_backend_data *backend = connssl->backend;
+
+  DEBUGASSERT(backend);
+
   if(!slot_name)
     return CURLE_OUT_OF_MEMORY;
 
@@ -1111,9 +1114,12 @@ static CURLcode cmp_peer_pubkey(struct ssl_connect_data *connssl,
 {
   CURLcode result = CURLE_SSL_PINNEDPUBKEYNOTMATCH;
   struct ssl_backend_data *backend = connssl->backend;
-  struct Curl_easy *data = backend->data;
+  struct Curl_easy *data = NULL;
   CERTCertificate *cert;
 
+  DEBUGASSERT(backend);
+  data = backend->data;
+
   if(!pinnedpubkey)
     /* no pinned public key specified */
     return CURLE_OK;
@@ -1164,10 +1170,15 @@ static SECStatus SelectClientCert(void *arg, PRFileDesc *sock,
 {
   struct ssl_connect_data *connssl = (struct ssl_connect_data *)arg;
   struct ssl_backend_data *backend = connssl->backend;
-  struct Curl_easy *data = backend->data;
-  const char *nickname = backend->client_nickname;
+  struct Curl_easy *data = NULL;
+  const char *nickname = NULL;
   static const char pem_slotname[] = "PEM Token #1";
 
+  DEBUGASSERT(backend);
+
+  data = backend->data;
+  nickname = backend->client_nickname;
+
   if(backend->obj_clicert) {
     /* use the cert/key provided by PEM reader */
     SECItem cert_der = { 0, NULL, 0 };
@@ -1535,6 +1546,8 @@ static int nss_check_cxn(struct connectdata *conn)
   int rc;
   char buf;
 
+  DEBUGASSERT(backend);
+
   rc =
     PR_Recv(backend->handle, (void *)&buf, 1, PR_MSG_PEEK,
             PR_SecondsToInterval(1));
@@ -1551,7 +1564,11 @@ static void close_one(struct ssl_connect_data *connssl)
 {
   /* before the cleanup, check whether we are using a client certificate */
   struct ssl_backend_data *backend = connssl->backend;
-  const bool client_cert = (backend->client_nickname != NULL)
+  bool client_cert = true;
+
+  DEBUGASSERT(backend);
+
+  client_cert = (backend->client_nickname != NULL)
     || (backend->obj_clicert != NULL);
 
   if(backend->handle) {
@@ -1593,8 +1610,13 @@ static void nss_close(struct Curl_easy *data, struct connectdata *conn,
   struct ssl_connect_data *connssl_proxy = &conn->proxy_ssl[sockindex];
 #endif
   struct ssl_backend_data *backend = connssl->backend;
-
   (void)data;
+
+  DEBUGASSERT(backend);
+#ifndef CURL_DISABLE_PROXY
+  DEBUGASSERT(connssl_proxy->backend != NULL);
+#endif
+
   if(backend->handle
 #ifndef CURL_DISABLE_PROXY
     || connssl_proxy->backend->handle
@@ -1822,6 +1844,8 @@ static CURLcode nss_fail_connect(struct ssl_connect_data *connssl,
 {
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   if(is_nss_error(curlerr)) {
     /* read NSPR error code */
     PRErrorCode err = PR_GetError();
@@ -1848,6 +1872,9 @@ static CURLcode nss_set_blocking(struct ssl_connect_data *connssl,
 {
   PRSocketOptionData sock_opt;
   struct ssl_backend_data *backend = connssl->backend;
+
+  DEBUGASSERT(backend);
+
   sock_opt.option = PR_SockOpt_Nonblocking;
   sock_opt.value.non_blocking = !blocking;
 
@@ -1889,6 +1916,8 @@ static CURLcode nss_setup_connect(struct Curl_easy *data,
     return CURLE_SSL_CONNECT_ERROR;
   }
 
+  DEBUGASSERT(backend);
+
   backend->data = data;
 
   /* list of all NSS objects we need to destroy in nss_do_close() */
@@ -2038,9 +2067,12 @@ static CURLcode nss_setup_connect(struct Curl_easy *data,
 
 #ifndef CURL_DISABLE_PROXY
   if(conn->proxy_ssl[sockindex].use) {
+    struct ssl_backend_data *proxy_backend;
+    proxy_backend = conn->proxy_ssl[sockindex].backend;
     DEBUGASSERT(ssl_connection_complete == conn->proxy_ssl[sockindex].state);
-    DEBUGASSERT(conn->proxy_ssl[sockindex].backend->handle != NULL);
-    nspr_io = conn->proxy_ssl[sockindex].backend->handle;
+    DEBUGASSERT(proxy_backend);
+    DEBUGASSERT(proxy_backend->handle);
+    nspr_io = proxy_backend->handle;
     second_layer = TRUE;
   }
 #endif
@@ -2182,6 +2214,8 @@ static CURLcode nss_do_connect(struct Curl_easy *data,
     goto error;
   }
 
+  DEBUGASSERT(backend);
+
   /* Force the handshake now */
   timeout = PR_MillisecondsToInterval((PRUint32) time_left);
   if(SSL_ForceHandshakeWithTimeout(backend->handle, timeout) != SECSuccess) {
@@ -2315,6 +2349,8 @@ static ssize_t nss_send(struct Curl_easy *data,    /* transfer */
   struct ssl_backend_data *backend = connssl->backend;
   ssize_t rc;
 
+  DEBUGASSERT(backend);
+
   /* The SelectClientCert() hook uses this for infof() and failf() but the
      handle stored in nss_setup_connect() could have already been freed. */
   backend->data = data;
@@ -2354,6 +2390,8 @@ static ssize_t nss_recv(struct Curl_easy *data,    /* transfer */
   struct ssl_backend_data *backend = connssl->backend;
   ssize_t nread;
 
+  DEBUGASSERT(backend);
+
   /* The SelectClientCert() hook uses this for infof() and failf() but the
      handle stored in nss_setup_connect() could have already been freed. */
   backend->data = data;
@@ -2452,6 +2490,7 @@ static void *nss_get_internals(struct ssl_connect_data *connssl,
 {
   struct ssl_backend_data *backend = connssl->backend;
   (void)info;
+  DEBUGASSERT(backend);
   return backend->handle;
 }
 
diff --git a/lib/vtls/openssl.c b/lib/vtls/openssl.c
index eaf7028f00..fc35f784da 100644
--- a/lib/vtls/openssl.c
+++ b/lib/vtls/openssl.c
@@ -1431,6 +1431,9 @@ static void ossl_closeone(struct Curl_easy *data,
                           struct ssl_connect_data *connssl)
 {
   struct ssl_backend_data *backend = connssl->backend;
+
+  DEBUGASSERT(backend);
+
   if(backend->handle) {
     char buf[32];
     set_logger(conn, data);
@@ -1488,6 +1491,8 @@ static int ossl_shutdown(struct Curl_easy *data,
   struct ssl_backend_data *backend = connssl->backend;
   int loop = 10;
 
+  DEBUGASSERT(backend);
+
 #ifndef CURL_DISABLE_FTP
   /* This has only been tested on the proftpd server, and the mod_tls code
      sends a close notify alert without waiting for a close notify alert in
@@ -1861,8 +1866,11 @@ static CURLcode verifystatus(struct Curl_easy *data,
   int cert_status, crl_reason;
   ASN1_GENERALIZEDTIME *rev, *thisupd, *nextupd;
   int ret;
+  long len;
 
-  long len = SSL_get_tlsext_status_ocsp_resp(backend->handle, &status);
+  DEBUGASSERT(backend);
+
+  len = SSL_get_tlsext_status_ocsp_resp(backend->handle, &status);
 
   if(!status) {
     failf(data, "No OCSP response received");
@@ -2121,7 +2129,10 @@ static void ossl_trace(int direction, int ssl_ver, int content_type,
   struct connectdata *conn = userp;
   struct ssl_connect_data *connssl = &conn->ssl[0];
   struct ssl_backend_data *backend = connssl->backend;
-  struct Curl_easy *data = backend->logger;
+  struct Curl_easy *data = NULL;
+
+  DEBUGASSERT(backend);
+  data = backend->logger;
 
   if(!conn || !data || !data->set.fdebug ||
      (direction != 0 && direction != 1))
@@ -2410,6 +2421,7 @@ set_ssl_version_min_max_legacy(ctx_option_t *ctx_options,
     {
       struct ssl_connect_data *connssl = &conn->ssl[sockindex];
       struct ssl_backend_data *backend = connssl->backend;
+      DEBUGASSERT(backend);
       SSL_CTX_set_max_proto_version(backend->ctx, TLS1_3_VERSION);
       *ctx_options |= SSL_OP_NO_TLSv1_2;
     }
@@ -2637,6 +2649,7 @@ static CURLcode ossl_connect_step1(struct Curl_easy *data,
   bool imported_native_ca = false;
 
   DEBUGASSERT(ssl_connect_1 == connssl->connecting_state);
+  DEBUGASSERT(backend);
 
   /* Make funny stuff to get random input */
   result = ossl_seed(data);
@@ -3245,7 +3258,11 @@ static CURLcode ossl_connect_step1(struct Curl_easy *data,
 #ifndef CURL_DISABLE_PROXY
   if(conn->proxy_ssl[sockindex].use) {
     BIO *const bio = BIO_new(BIO_f_ssl());
-    SSL *handle = conn->proxy_ssl[sockindex].backend->handle;
+    struct ssl_backend_data *proxy_backend;
+    SSL* handle = NULL;
+    proxy_backend = conn->proxy_ssl[sockindex].backend;
+    DEBUGASSERT(proxy_backend);
+    handle = proxy_backend->handle;
     DEBUGASSERT(ssl_connection_complete == conn->proxy_ssl[sockindex].state);
     DEBUGASSERT(handle != NULL);
     DEBUGASSERT(bio != NULL);
@@ -3275,6 +3292,7 @@ static CURLcode ossl_connect_step2(struct Curl_easy *data,
   DEBUGASSERT(ssl_connect_2 == connssl->connecting_state
               || ssl_connect_2_reading == connssl->connecting_state
               || ssl_connect_2_writing == connssl->connecting_state);
+  DEBUGASSERT(backend);
 
   ERR_clear_error();
 
@@ -3536,6 +3554,8 @@ static CURLcode get_cert_chain(struct Curl_easy *data,
   BIO *mem;
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   sk = SSL_get_peer_cert_chain(backend->handle);
   if(!sk) {
     return CURLE_OUT_OF_MEMORY;
@@ -3848,6 +3868,8 @@ static CURLcode servercert(struct Curl_easy *data,
   BIO *mem = BIO_new(BIO_s_mem());
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   if(!mem) {
     failf(data,
           "BIO_new return NULL, " OSSL_PACKAGE
@@ -4198,11 +4220,13 @@ static bool ossl_data_pending(const struct connectdata *conn,
                               int connindex)
 {
   const struct ssl_connect_data *connssl = &conn->ssl[connindex];
+  DEBUGASSERT(connssl->backend);
   if(connssl->backend->handle && SSL_pending(connssl->backend->handle))
     return TRUE;
 #ifndef CURL_DISABLE_PROXY
   {
     const struct ssl_connect_data *proxyssl = &conn->proxy_ssl[connindex];
+    DEBUGASSERT(proxyssl->backend);
     if(proxyssl->backend->handle && SSL_pending(proxyssl->backend->handle))
       return TRUE;
   }
@@ -4229,6 +4253,8 @@ static ssize_t ossl_send(struct Curl_easy *data,
   struct ssl_connect_data *connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   ERR_clear_error();
 
   memlen = (len > (size_t)INT_MAX) ? INT_MAX : (int)len;
@@ -4308,6 +4334,8 @@ static ssize_t ossl_recv(struct Curl_easy *data,   /* transfer */
   struct ssl_connect_data *connssl = &conn->ssl[num];
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   ERR_clear_error();
 
   buffsize = (buffersize > (size_t)INT_MAX) ? INT_MAX : (int)buffersize;
@@ -4507,6 +4535,7 @@ static void *ossl_get_internals(struct ssl_connect_data *connssl,
 {
   /* Legacy: CURLINFO_TLS_SESSION must return an SSL_CTX pointer. */
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
   return info == CURLINFO_TLS_SESSION ?
          (void *)backend->ctx : (void *)backend->handle;
 }
@@ -4517,6 +4546,7 @@ static bool ossl_associate_connection(struct Curl_easy *data,
 {
   struct ssl_connect_data *connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
 
   /* If we don't have SSL context, do nothing. */
   if(!backend->handle)
@@ -4566,6 +4596,7 @@ static void ossl_disassociate_connection(struct Curl_easy *data,
   struct connectdata *conn = data->conn;
   struct ssl_connect_data *connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
 
   /* If we don't have SSL context, do nothing. */
   if(!backend->handle)
diff --git a/lib/vtls/rustls.c b/lib/vtls/rustls.c
index 1c4cb19104..0e651aed9d 100644
--- a/lib/vtls/rustls.c
+++ b/lib/vtls/rustls.c
@@ -65,6 +65,7 @@ cr_data_pending(const struct connectdata *conn, int sockindex)
 {
   const struct ssl_connect_data *connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
   return backend->data_pending;
 }
 
@@ -118,7 +119,8 @@ cr_recv(struct Curl_easy *data, int sockindex,
   struct connectdata *conn = data->conn;
   struct ssl_connect_data *const connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *const backend = connssl->backend;
-  struct rustls_connection *const rconn = backend->conn;
+  struct rustls_connection *rconn = NULL;
+
   size_t n = 0;
   size_t tls_bytes_read = 0;
   size_t plain_bytes_copied = 0;
@@ -126,6 +128,9 @@ cr_recv(struct Curl_easy *data, int sockindex,
   char errorbuf[255];
   rustls_io_result io_error;
 
+  DEBUGASSERT(backend);
+  rconn = backend->conn;
+
   io_error = rustls_connection_read_tls(rconn, read_cb,
     &conn->sock[sockindex], &tls_bytes_read);
   if(io_error == EAGAIN || io_error == EWOULDBLOCK) {
@@ -215,13 +220,16 @@ cr_send(struct Curl_easy *data, int sockindex,
   struct connectdata *conn = data->conn;
   struct ssl_connect_data *const connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *const backend = connssl->backend;
-  struct rustls_connection *const rconn = backend->conn;
+  struct rustls_connection *rconn = NULL;
   size_t plainwritten = 0;
   size_t tlswritten = 0;
   size_t tlswritten_total = 0;
   rustls_result rresult;
   rustls_io_result io_error;
 
+  DEBUGASSERT(backend);
+  rconn = backend->conn;
+
   infof(data, "cr_send %ld bytes of plaintext", plainlen);
 
   if(plainlen > 0) {
@@ -295,7 +303,7 @@ static CURLcode
 cr_init_backend(struct Curl_easy *data, struct connectdata *conn,
                 struct ssl_backend_data *const backend)
 {
-  struct rustls_connection *rconn = backend->conn;
+  struct rustls_connection *rconn = NULL;
   struct rustls_client_config_builder *config_builder = NULL;
   struct rustls_root_cert_store *roots = NULL;
   const struct curl_blob *ca_info_blob = SSL_CONN_CONFIG(ca_info_blob);
@@ -312,6 +320,9 @@ cr_init_backend(struct Curl_easy *data, struct connectdata *conn,
     { (const uint8_t *)ALPN_H2, ALPN_H2_LENGTH },
   };
 
+  DEBUGASSERT(backend);
+  rconn = backend->conn;
+
   config_builder = rustls_client_config_builder_new();
 #ifdef USE_HTTP2
   infof(data, "offering ALPN for HTTP/1.1 and HTTP/2");
@@ -435,6 +446,8 @@ cr_connect_nonblocking(struct Curl_easy *data, struct connectdata *conn,
   curl_socket_t writefd;
   curl_socket_t readfd;
 
+  DEBUGASSERT(backend);
+
   if(ssl_connection_none == connssl->state) {
     result = cr_init_backend(data, conn, connssl->backend);
     if(result != CURLE_OK) {
@@ -529,7 +542,10 @@ cr_getsock(struct connectdata *conn, curl_socket_t *socks)
   struct ssl_connect_data *const connssl = &conn->ssl[FIRSTSOCKET];
   curl_socket_t sockfd = conn->sock[FIRSTSOCKET];
   struct ssl_backend_data *const backend = connssl->backend;
-  struct rustls_connection *rconn = backend->conn;
+  struct rustls_connection *rconn = NULL;
+
+  DEBUGASSERT(backend);
+  rconn = backend->conn;
 
   if(rustls_connection_wants_write(rconn)) {
     socks[0] = sockfd;
@@ -548,6 +564,7 @@ cr_get_internals(struct ssl_connect_data *connssl,
                  CURLINFO info UNUSED_PARAM)
 {
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
   return &backend->conn;
 }
 
@@ -560,6 +577,8 @@ cr_close(struct Curl_easy *data, struct connectdata *conn,
   CURLcode tmperr = CURLE_OK;
   ssize_t n = 0;
 
+  DEBUGASSERT(backend);
+
   if(backend->conn) {
     rustls_connection_send_close_notify(backend->conn);
     n = cr_send(data, sockindex, NULL, 0, &tmperr);
diff --git a/lib/vtls/schannel.c b/lib/vtls/schannel.c
index ae36630e74..04c8f3b6cf 100644
--- a/lib/vtls/schannel.c
+++ b/lib/vtls/schannel.c
@@ -426,6 +426,8 @@ schannel_acquire_credential_handle(struct Curl_easy *data,
   CURLcode result;
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   /* setup Schannel API options */
   memset(&schannel_cred, 0, sizeof(schannel_cred));
   schannel_cred.dwVersion = SCHANNEL_CRED_VERSION;
@@ -772,6 +774,8 @@ schannel_connect_step1(struct Curl_easy *data, struct connectdata *conn,
   char * const hostname = SSL_HOST_NAME();
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   DEBUGF(infof(data,
                "schannel: SSL/TLS connection with %s port %hu (step 1/3)",
                hostname, conn->remote_port));
@@ -1038,6 +1042,8 @@ schannel_connect_step2(struct Curl_easy *data, struct connectdata *conn,
   const char *pubkey_ptr;
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   doread = (connssl->connecting_state != ssl_connect_2_writing) ? TRUE : FALSE;
 
   DEBUGF(infof(data,
@@ -1371,6 +1377,7 @@ schannel_connect_step3(struct Curl_easy *data, struct connectdata *conn,
   struct ssl_backend_data *backend = connssl->backend;
 
   DEBUGASSERT(ssl_connect_3 == connssl->connecting_state);
+  DEBUGASSERT(backend);
 
   DEBUGF(infof(data,
                "schannel: SSL/TLS connection with %s port %hu (step 3/3)",
@@ -1611,6 +1618,7 @@ schannel_connect_common(struct Curl_easy *data, struct connectdata *conn,
      */
     {
       struct ssl_backend_data *backend = connssl->backend;
+      DEBUGASSERT(backend);
       conn->sslContext = &backend->ctxt->ctxt_handle;
     }
 #endif
@@ -1641,6 +1649,8 @@ schannel_send(struct Curl_easy *data, int sockindex,
   CURLcode result;
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   /* check if the maximum stream sizes were queried */
   if(backend->stream_sizes.cbMaximumMessage == 0) {
     sspi_status = s_pSecFn->QueryContextAttributes(
@@ -1789,6 +1799,8 @@ schannel_recv(struct Curl_easy *data, int sockindex,
   size_t min_encdata_length = len + CURL_SCHANNEL_BUFFER_FREE_SIZE;
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   /****************************************************************************
    * Don't return or set backend->recv_unrecoverable_err unless in the cleanup.
    * The pattern for return error is set *err, optional infof, goto cleanup.
@@ -2123,6 +2135,8 @@ static bool schannel_data_pending(const struct connectdata *conn,
   const struct ssl_connect_data *connssl = &conn->ssl[sockindex];
   struct ssl_backend_data *backend = connssl->backend;
 
+  DEBUGASSERT(backend);
+
   if(connssl->use) /* SSL/TLS is in use */
     return (backend->decdata_offset > 0 ||
             (backend->encdata_offset > 0 && !backend->encdata_is_incomplete));
@@ -2159,6 +2173,7 @@ static int schannel_shutdown(struct Curl_easy *data, struct connectdata *conn,
   struct ssl_backend_data *backend = connssl->backend;
 
   DEBUGASSERT(data);
+  DEBUGASSERT(backend);
 
   if(connssl->use) {
     infof(data, "schannel: shutting down SSL/TLS connection with %s port %hu",
@@ -2309,6 +2324,8 @@ static CURLcode pkp_pin_peer_pubkey(struct Curl_easy *data,
   /* Result is returned to caller */
   CURLcode result = CURLE_SSL_PINNEDPUBKEYNOTMATCH;
 
+  DEBUGASSERT(backend);
+
   /* if a path wasn't specified, don't pin */
   if(!pinnedpubkey)
     return CURLE_OK;
@@ -2429,6 +2446,7 @@ static void *schannel_get_internals(struct ssl_connect_data *connssl,
 {
   struct ssl_backend_data *backend = connssl->backend;
   (void)info;
+  DEBUGASSERT(backend);
   return &backend->ctxt->ctxt_handle;
 }
 
diff --git a/lib/vtls/schannel_verify.c b/lib/vtls/schannel_verify.c
index 01de3fc732..202a814cd9 100644
--- a/lib/vtls/schannel_verify.c
+++ b/lib/vtls/schannel_verify.c
@@ -576,6 +576,8 @@ CURLcode Curl_verify_certificate(struct Curl_easy *data,
   HCERTSTORE trust_store = NULL;
   const char * const conn_hostname = SSL_HOST_NAME();
 
+  DEBUGASSERT(BACKEND);
+
   sspi_status =
     s_pSecFn->QueryContextAttributes(&BACKEND->ctxt->ctxt_handle,
                                      SECPKG_ATTR_REMOTE_CERT_CONTEXT,
diff --git a/lib/vtls/sectransp.c b/lib/vtls/sectransp.c
index be41d4f4bb..b2e1727278 100644
--- a/lib/vtls/sectransp.c
+++ b/lib/vtls/sectransp.c
@@ -837,12 +837,14 @@ static OSStatus SocketRead(SSLConnectionRef connection,
   /*int sock = *(int *)connection;*/
   struct ssl_connect_data *connssl = (struct ssl_connect_data *)connection;
   struct ssl_backend_data *backend = connssl->backend;
-  int sock = backend->ssl_sockfd;
+  int sock;
   OSStatus rtn = noErr;
   size_t bytesRead;
   ssize_t rrtn;
   int theErr;
 
+  DEBUGASSERT(backend);
+  sock = backend->ssl_sockfd;
   *dataLength = 0;
 
   for(;;) {
@@ -898,13 +900,15 @@ static OSStatus SocketWrite(SSLConnectionRef connection,
   /*int sock = *(int *)connection;*/
   struct ssl_connect_data *connssl = (struct ssl_connect_data *)connection;
   struct ssl_backend_data *backend = connssl->backend;
-  int sock = backend->ssl_sockfd;
+  int sock;
   ssize_t length;
   size_t dataLen = *dataLength;
   const UInt8 *dataPtr = (UInt8 *)data;
   OSStatus ortn;
   int theErr;
 
+  DEBUGASSERT(backend);
+  sock = backend->ssl_sockfd;
   *dataLength = 0;
 
   do {
@@ -1376,6 +1380,8 @@ set_ssl_version_min_max(struct Curl_easy *data, struct connectdata *conn,
   long ssl_version_max = SSL_CONN_CONFIG(version_max);
   long max_supported_version_by_os;
 
+  DEBUGASSERT(backend);
+
   /* macOS 10.5-10.7 supported TLS 1.0 only.
      macOS 10.8 and later, and iOS 5 and later, added TLS 1.1 and 1.2.
      macOS 10.13 and later, and iOS 11 and later, added TLS 1.3. */
@@ -1684,6 +1690,8 @@ static CURLcode sectransp_connect_step1(struct Curl_easy *data,
 #if CURL_BUILD_MAC
   int darwinver_maj = 0, darwinver_min = 0;
 
+  DEBUGASSERT(backend);
+
   GetDarwinVersionNumber(&darwinver_maj, &darwinver_min);
 #endif /* CURL_BUILD_MAC */
 
@@ -2547,6 +2555,7 @@ sectransp_connect_step2(struct Curl_easy *data, struct connectdata *conn,
   DEBUGASSERT(ssl_connect_2 == connssl->connecting_state
               || ssl_connect_2_reading == connssl->connecting_state
               || ssl_connect_2_writing == connssl->connecting_state);
+  DEBUGASSERT(backend);
 
   /* Here goes nothing: */
   err = SSLHandshake(backend->ssl_ctx);
@@ -2923,6 +2932,8 @@ collect_server_cert(struct Curl_easy *data,
   CFIndex i, count;
   SecTrustRef trust = NULL;
 
+  DEBUGASSERT(backend);
+
   if(!show_verbose_server_cert && !data->set.ssl.certinfo)
     return CURLE_OK;
 
@@ -3167,6 +3178,8 @@ static void sectransp_close(struct Curl_easy *data, struct connectdata *conn,
 
   (void) data;
 
+  DEBUGASSERT(backend);
+
   if(backend->ssl_ctx) {
     (void)SSLClose(backend->ssl_ctx);
 #if CURL_BUILD_MAC_10_8 || CURL_BUILD_IOS
@@ -3195,6 +3208,8 @@ static int sectransp_shutdown(struct Curl_easy *data,
   char buf[120];
   int loop = 10; /* avoid getting stuck */
 
+  DEBUGASSERT(backend);
+
   if(!backend->ssl_ctx)
     return 0;
 
@@ -3274,6 +3289,8 @@ static int sectransp_check_cxn(struct connectdata *conn)
   OSStatus err;
   SSLSessionState state;
 
+  DEBUGASSERT(backend);
+
   if(backend->ssl_ctx) {
     err = SSLGetSessionState(backend->ssl_ctx, &state);
     if(err == noErr)
@@ -3291,6 +3308,8 @@ static bool sectransp_data_pending(const struct connectdata *conn,
   OSStatus err;
   size_t buffer;
 
+  DEBUGASSERT(backend);
+
   if(backend->ssl_ctx) {  /* SSL is in use */
     err = SSLGetBufferedReadSize(backend->ssl_ctx, &buffer);
     if(err == noErr)
@@ -3352,6 +3371,8 @@ static ssize_t sectransp_send(struct Curl_easy *data,
   size_t processed = 0UL;
   OSStatus err;
 
+  DEBUGASSERT(backend);
+
   /* The SSLWrite() function works a little differently than expected. The
      fourth argument (processed) is currently documented in Apple's
      documentation as: "On return, the length, in bytes, of the data actually
@@ -3419,6 +3440,8 @@ static ssize_t sectransp_recv(struct Curl_easy *data,
   size_t processed = 0UL;
   OSStatus err;
 
+  DEBUGASSERT(backend);
+
   again:
   err = SSLRead(backend->ssl_ctx, buf, buffersize, &processed);
 
@@ -3468,6 +3491,7 @@ static void *sectransp_get_internals(struct ssl_connect_data *connssl,
 {
   struct ssl_backend_data *backend = connssl->backend;
   (void)info;
+  DEBUGASSERT(backend);
   return backend->ssl_ctx;
 }
 
diff --git a/lib/vtls/vtls.c b/lib/vtls/vtls.c
index 41714d54b2..bf7a9a1e92 100644
--- a/lib/vtls/vtls.c
+++ b/lib/vtls/vtls.c
@@ -300,6 +300,8 @@ ssl_connect_init_proxy(struct connectdata *conn, int sockindex)
     pbdata = conn->proxy_ssl[sockindex].backend;
     conn->proxy_ssl[sockindex] = conn->ssl[sockindex];
 
+    DEBUGASSERT(pbdata != NULL);
+
     memset(&conn->ssl[sockindex], 0, sizeof(conn->ssl[sockindex]));
     memset(pbdata, 0, Curl_ssl->sizeof_ssl_backend_data);
 
diff --git a/lib/vtls/wolfssl.c b/lib/vtls/wolfssl.c
index 1dbf5ecbb5..eae5568309 100644
--- a/lib/vtls/wolfssl.c
+++ b/lib/vtls/wolfssl.c
@@ -263,6 +263,8 @@ wolfssl_connect_step1(struct Curl_easy *data, struct connectdata *conn,
 #define use_sni(x)  Curl_nop_stmt
 #endif
 
+  DEBUGASSERT(backend);
+
   if(connssl->state == ssl_connection_complete)
     return CURLE_OK;
 
@@ -598,6 +600,8 @@ wolfssl_connect_step2(struct Curl_easy *data, struct connectdata *conn,
   const char * const dispname = SSL_HOST_DISPNAME();
   const char * const pinnedpubkey = SSL_PINNED_PUB_KEY();
 
+  DEBUGASSERT(backend);
+
   ERR_clear_error();
 
   conn->recv[sockindex] = wolfssl_recv;
@@ -802,6 +806,7 @@ wolfssl_connect_step3(struct Curl_easy *data, struct connectdata *conn,
   struct ssl_backend_data *backend = connssl->backend;
 
   DEBUGASSERT(ssl_connect_3 == connssl->connecting_state);
+  DEBUGASSERT(backend);
 
   if(SSL_SET_OPTION(primary.sessionid)) {
     bool incache;
@@ -853,6 +858,8 @@ static ssize_t wolfssl_send(struct Curl_easy *data,
   int memlen = (len > (size_t)INT_MAX) ? INT_MAX : (int)len;
   int rc;
 
+  DEBUGASSERT(backend);
+
   ERR_clear_error();
 
   rc = SSL_write(backend->handle, mem, memlen);
@@ -885,6 +892,8 @@ static void wolfssl_close(struct Curl_easy *data, struct connectdata *conn,
 
   (void) data;
 
+  DEBUGASSERT(backend);
+
   if(backend->handle) {
     char buf[32];
     /* Maybe the server has already sent a close notify alert.
@@ -913,6 +922,8 @@ static ssize_t wolfssl_recv(struct Curl_easy *data,
   int buffsize = (buffersize > (size_t)INT_MAX) ? INT_MAX : (int)buffersize;
   int nread;
 
+  DEBUGASSERT(backend);
+
   ERR_clear_error();
 
   nread = SSL_read(backend->handle, buf, buffsize);
@@ -982,6 +993,7 @@ static bool wolfssl_data_pending(const struct connectdata *conn,
 {
   const struct ssl_connect_data *connssl = &conn->ssl[connindex];
   struct ssl_backend_data *backend = connssl->backend;
+  DEBUGASSERT(backend);
   if(backend->handle)   /* SSL is in use */
     return (0 != SSL_pending(backend->handle)) ? TRUE : FALSE;
   else
@@ -1002,6 +1014,8 @@ static int wolfssl_shutdown(struct Curl_easy *data, struct connectdata *conn,
 
   (void) data;
 
+  DEBUGASSERT(backend);
+
   if(backend->handle) {
     ERR_clear_error();
     SSL_free(backend->handle);
@@ -1181,6 +1195,7 @@ static void *wolfssl_get_internals(struct ssl_connect_data *connssl,
 {
   struct ssl_backend_data *backend = connssl->backend;
   (void)info;
+  DEBUGASSERT(backend);
   return backend->handle;
 }