Index: Python/Python-ast.c
===================================================================
--- Python/Python-ast.c	(revision 62050)
+++ Python/Python-ast.c	(working copy)
@@ -378,7 +378,7 @@
     fields = PyObject_GetAttrString((PyObject*)Py_TYPE(self), "_fields");
     if (!fields)
         PyErr_Clear();
-    if (fields) {
+    if (fields && fields != Py_None) {
         numfields = PySequence_Size(fields);
         if (numfields == -1)
             goto cleanup;
@@ -388,7 +388,7 @@
         if (numfields != PyTuple_GET_SIZE(args)) {
             PyErr_Format(PyExc_TypeError, "%.400s constructor takes either 0 or "
                          "%d positional argument%s", Py_TYPE(self)->tp_name,
-                         numfields, numfields == 1 ? "" : "s");
+                         (int)numfields, numfields == 1 ? "" : "s");
             res = -1;
             goto cleanup;
         }