diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
index 29804f5..0309f1de 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -1806,7 +1806,7 @@
 	PyObject *result = NULL;
 	PyObject *temp, *item, *iter;
 
-	if (!PyArg_ParseTuple(args, "O|O:sum", &seq, &result))
+	if (!PyArg_UnpackTuple(args, "sum", 1, 2, &seq, &result))
 		return NULL;
 
 	iter = PyObject_GetIter(seq);