diff --git a/psycopg/connection_int.c b/psycopg/connection_int.c
index d92d53f7..2f2a48d6 100644
--- a/psycopg/connection_int.c
+++ b/psycopg/connection_int.c
@@ -939,21 +939,30 @@ conn_tpc_command(connectionObject *self, const char *cmd, XidObject *xid)
 {
     PGresult *pgres = NULL;
     char *error = NULL;
-    int rv;
+    char *tid = NULL;
+    int rv = -1;
 
     Dprintf("conn_tpc_command: %s", cmd);
 
+    /* convert the xid into PostgreSQL transaction id while keeping the GIL */
+    if (!(tid = xid_get_tid(xid))) { goto exit; }
+
     Py_BEGIN_ALLOW_THREADS;
     pthread_mutex_lock(&self->lock);
 
-    rv = pq_tpc_command_locked(self, cmd, xid, &pgres, &error, &_save);
+    if (0 > (rv = pq_tpc_command_locked(self, cmd, tid,
+                                        &pgres, &error, &_save))) {
+        pthread_mutex_unlock(&self->lock);
+        Py_BLOCK_THREADS;
+        pq_complete_error(self, &pgres, &error);
+        goto exit;
+    }
 
     pthread_mutex_unlock(&self->lock);
     Py_END_ALLOW_THREADS;
 
-    if (rv < 0) {
-        pq_complete_error(self, &pgres, &error);
-    }
+exit:
+    PyMem_Free(tid);
     return rv;
 }
 
diff --git a/psycopg/pqpath.c b/psycopg/pqpath.c
index 37d52791..3977f31d 100644
--- a/psycopg/pqpath.c
+++ b/psycopg/pqpath.c
@@ -617,19 +617,17 @@ pq_reset(connectionObject *conn)
  * holding the global interpreter lock. */
 
 int
-pq_tpc_command_locked(connectionObject *conn, const char *cmd, XidObject *xid,
-                  PGresult **pgres, char **error,
-                  PyThreadState **tstate)
+pq_tpc_command_locked(connectionObject *conn, const char *cmd, const char *tid,
+                  PGresult **pgres, char **error, PyThreadState **tstate)
 {
     int rv = -1;
-    char *tid = NULL, *etid = NULL, *buf = NULL;
+    char *etid = NULL, *buf = NULL;
     Py_ssize_t buflen;
 
     Dprintf("_pq_tpc_command: pgconn = %p, command = %s",
             conn->pgconn, cmd);
 
     /* convert the xid into the postgres transaction_id and quote it. */
-    if (!(tid = xid_get_tid(xid))) { goto exit; }
     if (!(etid = psycopg_escape_string((PyObject *)conn, tid, 0, NULL, NULL)))
     { goto exit; }
 
@@ -647,7 +645,6 @@ pq_tpc_command_locked(connectionObject *conn, const char *cmd, XidObject *xid,
 exit:
     PyMem_Free(buf);
     PyMem_Free(etid);
-    PyMem_Free(tid);
 
     return rv;
 }
diff --git a/psycopg/pqpath.h b/psycopg/pqpath.h
index 5f56efa4..51a640dc 100644
--- a/psycopg/pqpath.h
+++ b/psycopg/pqpath.h
@@ -47,7 +47,7 @@ HIDDEN int pq_abort_locked(connectionObject *conn, PGresult **pgres,
 HIDDEN int pq_abort(connectionObject *conn);
 HIDDEN int pq_reset(connectionObject *conn);
 HIDDEN int pq_tpc_command_locked(connectionObject *conn,
-                                 const char *cmd, XidObject *xid,
+                                 const char *cmd, const char *tid,
                                  PGresult **pgres, char **error,
                                  PyThreadState **tstate);
 HIDDEN int pq_is_busy(connectionObject *conn);