diff --git a/mozilla/js/src/jslock.c b/mozilla/js/src/jslock.c
index 54b4da4bf90..0a0e126c830 100644
--- a/mozilla/js/src/jslock.c
+++ b/mozilla/js/src/jslock.c
@@ -1242,6 +1242,15 @@ js_LockObj(JSContext *cx, JSObject *obj)
     JSScope *scope;
 
     JS_ASSERT(OBJ_IS_NATIVE(obj));
+
+    /*
+     * We must test whether the GC is calling and return without mutating any
+     * state, especially cx->lockedSealedScope.  Note asymmetry with respect to
+     * js_UnlockObj, which is a thin-layer on top of js_UnlockScope.
+     */
+    if (CX_THREAD_IS_RUNNING_GC(cx))
+        return;
+
     for (;;) {
         scope = OBJ_SCOPE(obj);
         if (SCOPE_IS_SEALED(scope) && scope->object == obj &&