diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index eb3560b..fabdd9d 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -1216,6 +1216,7 @@
 PyObject *
 _PyObject_LookupSpecial(PyObject *self, char *attrstr, PyObject **attrobj)
 {
+	assert(!PyInstance_Check(self));
 	return lookup_maybe(self, attrstr, attrobj);
 }