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-rw-r--r--dom/base/CustomElementRegistry.cpp2
1 files changed, 2 insertions, 0 deletions
diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index fc6ac2877..200668f9e 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -1045,6 +1045,8 @@ CustomElementReactionsStack::Enqueue(Element* aElement,
// If the custom element reactions stack is empty, then:
// Add element to the backup element queue.
+ MOZ_ASSERT(!aReaction->IsUpgradeReaction(),
+ "Upgrade reaction should not be scheduled to backup queue");
mBackupQueue.AppendElement(aElement);
elementData->mReactionQueue.AppendElement(aReaction);