diff --git a/mozilla/js/src/jsobj.c b/mozilla/js/src/jsobj.c
index 425b4fa2d99..3db67c800ba 100644
--- a/mozilla/js/src/jsobj.c
+++ b/mozilla/js/src/jsobj.c
@@ -3460,6 +3460,8 @@ js_FindPropertyHelper(JSContext *cx, jsid id, JSObject **objp,
         if (obj->map->ops->lookupProperty == js_LookupProperty) {
             protoIndex =
                 js_LookupPropertyWithFlags(cx, obj, id, 0, &pobj, &prop);
+            if (protoIndex < 0)
+                return -1;
         } else {
             if (!OBJ_LOOKUP_PROPERTY(cx, obj, id, &pobj, &prop))
                 return -1;