diff --git a/psycopg/xid.h b/psycopg/xid.h
index 598d6a53..0c0d3bd3 100644
--- a/psycopg/xid.h
+++ b/psycopg/xid.h
@@ -31,9 +31,6 @@
 
 #include "psycopg/config.h"
 
-/* value for the format_id when the xid doesn't follow the XA standard. */
-#define XID_UNPARSED (-2)
-
 extern HIDDEN PyTypeObject XidType;
 
 typedef struct {
diff --git a/psycopg/xid_type.c b/psycopg/xid_type.c
index e4d826ea..cfe0bc46 100644
--- a/psycopg/xid_type.c
+++ b/psycopg/xid_type.c
@@ -303,25 +303,13 @@ _xid_encode64(PyObject *s)
 {
     PyObject *base64 = NULL;
     PyObject *encode = NULL;
-    PyObject *out = NULL;
     PyObject *rv = NULL;
 
     if (!(base64 = PyImport_ImportModule("base64"))) { goto exit; }
     if (!(encode = PyObject_GetAttrString(base64, "b64encode"))) { goto exit; }
-    if (!(out = PyObject_CallFunctionObjArgs(encode, s, NULL))) { goto exit; }
-
-    /* we are going to use PyString_AS_STRING on this so let's ensure it. */
-    if (!PyString_Check(out)) {
-        PyErr_SetString(PyExc_TypeError,
-            "base64.b64encode didn't return a string");
-        goto exit;
-    }
-
-    rv = out;
-    out = NULL;
+    if (!(rv = PyObject_CallFunctionObjArgs(encode, s, NULL))) { goto exit; }
 
 exit:
-    Py_XDECREF(out);
     Py_XDECREF(encode);
     Py_XDECREF(base64);
 
@@ -364,16 +352,15 @@ char *
 xid_get_tid(XidObject *self)
 {
     char *buf = NULL;
-    long format_id;
     Py_ssize_t bufsize = 0;
     PyObject *egtrid = NULL;
     PyObject *ebqual = NULL;
+    PyObject *format = NULL;
+    PyObject *args = NULL;
     PyObject *tid = NULL;
 
-    format_id = PyInt_AsLong(self->format_id);
-    if (-1 == format_id && PyErr_Occurred()) { goto exit; }
-
-    if (XID_UNPARSED == format_id) {
+    if (Py_None == self->format_id) {
+        /* Unparsed xid: return the gtrid. */
         bufsize = 1 + PyString_Size(self->gtrid);
         if (!(buf = (char *)PyMem_Malloc(bufsize))) {
             PyErr_NoMemory();
@@ -382,23 +369,32 @@ xid_get_tid(XidObject *self)
         strncpy(buf, PyString_AsString(self->gtrid), bufsize);
     }
     else {
+        /* XA xid: mash together the components. */
         if (!(egtrid = _xid_encode64(self->gtrid))) { goto exit; }
         if (!(ebqual = _xid_encode64(self->bqual))) { goto exit; }
-        if (!(tid = PyString_FromFormat("%ld_%s_%s",
-                format_id,
-                PyString_AS_STRING(egtrid),
-                PyString_AS_STRING(ebqual)))) {
-            goto exit;
-        }
+
+        /* tid = "%d_%s_%s" % (format_id, egtrid, ebqual) */
+        if (!(format = PyString_FromString("%d_%s_%s"))) { goto exit; }
+
+        if (!(args = PyTuple_New(3))) { goto exit; }
+        Py_INCREF(self->format_id);
+        PyTuple_SET_ITEM(args, 0, self->format_id);
+        PyTuple_SET_ITEM(args, 1, egtrid); egtrid = NULL;
+        PyTuple_SET_ITEM(args, 2, ebqual); ebqual = NULL;
+
+        if (!(tid = PyString_Format(format, args))) { goto exit; }
+
         bufsize = 1 + PyString_Size(tid);
         if (!(buf = (char *)PyMem_Malloc(bufsize))) {
             PyErr_NoMemory();
             goto exit;
         }
-        strncpy(buf, PyString_AS_STRING(tid), bufsize);
+        strncpy(buf, PyString_AsString(tid), bufsize);
     }
 
 exit:
+    Py_XDECREF(args);
+    Py_XDECREF(format);
     Py_XDECREF(egtrid);
     Py_XDECREF(ebqual);
     Py_XDECREF(tid);
@@ -505,29 +501,27 @@ static XidObject *
 _xid_unparsed_from_string(PyObject *str) {
     XidObject *xid = NULL;
     XidObject *rv = NULL;
-    PyObject *format_id = NULL;
     PyObject *tmp;
 
     /* fake args to work around the checks performed by the xid init */
     if (!(xid = (XidObject *)PyObject_CallFunction((PyObject *)&XidType,
-            "iss", 0, "tmp", "tmp"))) {
+            "iss", 0, "", ""))) {
         goto exit;
     }
 
-    /* set xid.gtrid */
+    /* set xid.gtrid = str */
     tmp = xid->gtrid;
     Py_INCREF(str);
     xid->gtrid = str;
     Py_DECREF(tmp);
 
-    /* set xid.format_id */
-    if (!(format_id = PyInt_FromLong(XID_UNPARSED))) { goto exit; }
+    /* set xid.format_id = None */
     tmp = xid->format_id;
-    xid->format_id = format_id;
-    format_id = NULL;
+    Py_INCREF(Py_None);
+    xid->format_id = Py_None;
     Py_DECREF(tmp);
 
-    /* set xid.bqual */
+    /* set xid.bqual = None */
     tmp = xid->bqual;
     Py_INCREF(Py_None);
     xid->bqual = Py_None;
@@ -538,7 +532,6 @@ _xid_unparsed_from_string(PyObject *str) {
     xid = NULL;
 
 exit:
-    Py_XDECREF(format_id);
     Py_XDECREF(xid);
 
     return rv;
diff --git a/tests/test_connection.py b/tests/test_connection.py
index 9cefddeb..57bdd8ac 100644
--- a/tests/test_connection.py
+++ b/tests/test_connection.py
@@ -347,7 +347,7 @@ class ConnectionTwoPhaseTests(unittest.TestCase):
                 if xid.database == tests.dbname ]
             self.assertEqual(1, len(xids))
             xid = xids[0]
-            self.assertEqual(xid.format_id, -2)
+            self.assertEqual(xid.format_id, None)
             self.assertEqual(xid.gtrid, tid)
             self.assertEqual(xid.bqual, None)