Index: Objects/abstract.c
===================================================================
--- Objects/abstract.c	(revision 61567)
+++ Objects/abstract.c	(working copy)
@@ -2909,8 +2909,12 @@
 	static PyObject *name = NULL;
 	PyObject *t, *v, *tb;
 	PyObject *checker;
+
+	/* Quick test for an exact match */
+	if (Py_TYPE(inst) == cls && !PyTuple_Check(cls))
+		return 1;
+
 	PyErr_Fetch(&t, &v, &tb);
-
 	if (name == NULL) {
 		name = PyString_InternFromString("__instancecheck__");
 		if (name == NULL)