diff --git a/Objects/listobject.c b/Objects/listobject.c
index ed28200..727c9e6 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -475,7 +475,7 @@
 		if (a == b) {
 			/* Special case "a[i:j] = a" -- copy b first */
 			int ret;
-			v = list_slice(b, 0, n);
+			v = list_slice(b, 0, b->ob_size);
 			if (v == NULL)
 				return -1;
 			ret = list_ass_slice(a, ilow, ihigh, v);