Index: sliceobject.c
===================================================================
--- sliceobject.c	(revision 63787)
+++ sliceobject.c	(working copy)
@@ -169,7 +169,7 @@
 	else {
 		if (!_PyEval_SliceIndex(r->stop, stop)) return -1;
 		if (*stop < 0) *stop += length;
-		if (*stop < 0) *stop = -1;
+		if (*stop < 0) *stop = *step < 0 ? -1 : 0;
 		if (*stop > length) *stop = length;
 	}