diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 35dc76f..f15b096 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -273,6 +273,9 @@
 {
 	PyObject *mro;
 
+	if (!(a->tp_flags & Py_TPFLAGS_HAVE_CLASS))
+		return b == a || b == &PyBaseObject_Type;
+
 	mro = a->tp_mro;
 	if (mro != NULL) {
 		/* Deal with multiple inheritance without recursion