--- Python-2.5.2/Objects/dictobject.c	2007-11-29 18:33:04.000000000 +0000
+++ Python-2.5.2.new/Objects/dictobject.c	2008-07-28 13:34:09.000000000 +0100
@@ -1356,7 +1356,7 @@
 		return -1;
 	}
 	mp = (dictobject*)a;
-	if (PyDict_Check(b)) {
+	if (PyDict_CheckExact(b)) {
 		other = (dictobject*)b;
 		if (other == mp || other->ma_used == 0)
 			/* a.update(a) or a.update({}); nothing to do */