--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -1056,16 +1056,18 @@ CustomElementReactionsStack::Enqueue(Ele
     elementData->mReactionQueue.AppendElement(aReaction);
     return;
   }
 
   // If the custom element reactions stack is empty, then:
   // Add element to the backup element queue.
   MOZ_ASSERT(mReactionsStack.IsEmpty(),
              "custom element reactions stack should be empty");
+  MOZ_ASSERT(!GetIncumbentGlobal(),
+             "Custom element reaction should not be scheduled to backup queue if it is from author code");
   MOZ_ASSERT(!aReaction->IsUpgradeReaction(),
              "Upgrade reaction should not be scheduled to backup queue");
   mBackupQueue.AppendElement(aElement);
   elementData->mReactionQueue.AppendElement(aReaction);
 
   if (mIsBackupQueueProcessing) {
     return;
   }