--- a/dom/base/nsWrapperCache.h
+++ b/dom/base/nsWrapperCache.h
@@ -118,18 +118,20 @@ public:
   /**
    * Update the wrapper if the object it contains is moved.
    *
    * This method must be called from the objectMovedOp class extension hook for
    * any wrapper cached object.
    */
   void UpdateWrapper(JSObject* aNewObject, const JSObject* aOldObject)
   {
-    MOZ_ASSERT(mWrapper == aOldObject);
-    mWrapper = aNewObject;
+    if (mWrapper) {
+      MOZ_ASSERT(mWrapper == aOldObject);
+      mWrapper = aNewObject;
+    }
   }
 
   bool PreservingWrapper()
   {
     return HasWrapperFlag(WRAPPER_BIT_PRESERVED);
   }
 
   void SetIsDOMBinding()