diff --git a/dom/base/nsWrapperCache.h b/dom/base/nsWrapperCache.h
index f93e60684ea..75224af6975 100644
--- a/dom/base/nsWrapperCache.h
+++ b/dom/base/nsWrapperCache.h
@@ -123,8 +123,10 @@ public:
    */
   void UpdateWrapper(JSObject* aNewObject, const JSObject* aOldObject)
   {
-    MOZ_ASSERT(mWrapper == aOldObject);
-    mWrapper = aNewObject;
+    if (mWrapper) {
+      MOZ_ASSERT(mWrapper == aOldObject);
+      mWrapper = aNewObject;
+    }
   }
 
   bool PreservingWrapper()