--- a/dom/workers/XMLHttpRequest.cpp
+++ b/dom/workers/XMLHttpRequest.cpp
@@ -1878,16 +1878,21 @@ XMLHttpRequest::SendInternal(const nsASt
   mProxy->mOuterChannelId++;
 
   JSContext* cx = mWorkerPrivate->GetJSContext();
 
   nsRefPtr<SendRunnable> runnable =
     new SendRunnable(mWorkerPrivate, mProxy, aStringBody, Move(aBody),
                      aClonedObjects, syncLoopTarget, hasUploadListeners);
   if (!runnable->Dispatch(cx)) {
+    // Dispatch() may have spun the event loop and we may have already unrooted.
+    // If so we don't want autoUnpin to try again.
+    if (!mRooted) {
+      autoUnpin.Clear();
+    }
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
 
   if (!isSyncXHR)  {
     autoUnpin.Clear();
     MOZ_ASSERT(!autoSyncLoop);
     return;