Index: Python/ceval.c
===================================================================
--- Python/ceval.c	(revision 60729)
+++ Python/ceval.c	(working copy)
@@ -2684,7 +2684,7 @@
 			PyObject *keyword = kws[2*i];
 			PyObject *value = kws[2*i + 1];
 			int j;
-			if (keyword == NULL || !PyString_Check(keyword)) {
+			if (keyword == NULL || !PyString_CheckExact(keyword)) {
 				PyErr_Format(PyExc_TypeError,
 				    "%.200s() keywords must be strings",
 				    PyString_AsString(co->co_name));