diff --git a/dom/workers/XMLHttpRequest.cpp b/dom/workers/XMLHttpRequest.cpp
index ff316b88dfa..60df050bcd5 100644
--- a/dom/workers/XMLHttpRequest.cpp
+++ b/dom/workers/XMLHttpRequest.cpp
@@ -1279,6 +1279,10 @@ EventRunnable::WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate)
       mProxy->mSeenUploadLoadStart = false;
     }
     else {
+      if (!mProxy->mSeenLoadStart) {
+        // We've already dispatched premature abort events.
+        return true;
+      }
       mProxy->mSeenLoadStart = false;
     }
   }
@@ -2404,7 +2408,9 @@ XMLHttpRequest::UpdateState(const StateData& aStateData,
                             bool aUseCachedArrayBufferResponse)
 {
   if (aUseCachedArrayBufferResponse) {
-    MOZ_ASSERT(JS_IsArrayBufferObject(mStateData.mResponse.toObjectOrNull()));
+    MOZ_ASSERT(mStateData.mResponse.isObject() &&
+               JS_IsArrayBufferObject(&mStateData.mResponse.toObject()));
+
     JS::Rooted<JS::Value> response(mWorkerPrivate->GetJSContext(),
                                    mStateData.mResponse);
     mStateData = aStateData;