Minor fixes / changes for Mac compatibility.
diff --git a/Objects/listobject.c b/Objects/listobject.c
index b51e3d8..38829e2 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -442,7 +442,7 @@
 {
 	if (!is_listobject(a)) {
 		err_badcall();
-		return NULL;
+		return -1;
 	}
 	return list_ass_slice((listobject *)a, ilow, ihigh, v);
 }