Prevent a NULL pointer from being pushed onto the stack. It's possible for PyErr_NormalizeException() to set the traceback pointer to NULL. I'm not sure how to provoke this directly from Python, although it may be possible. The error occurs when an exception is set using PyErr_SetObject() and another exception occurs while PyErr_NormalizeException() is creating the exception instance. XXX As a result of this change, it's possible for an exception to occur but sys.last_traceback to be left undefined. Not sure if this is a problem.

--- a/Python/ceval.c	Wed Sep 26 18:46:36 2001 +0000
+++ b/Python/ceval.c	Wed Sep 26 19:24:45 2001 +0000
@@ -2255,7 +2255,11 @@
 						set_exc_info(tstate,
 							     exc, val, tb);
 					}
-					PUSH(tb);
+					if (tb == NULL) {
+						Py_INCREF(Py_None);
+						PUSH(Py_None);
+					} else
+						PUSH(tb);
 					PUSH(val);
 					PUSH(exc);
 				}