--- a/dom/media/mediasource/TrackBuffersManager.cpp
+++ b/dom/media/mediasource/TrackBuffersManager.cpp
@@ -860,19 +860,22 @@ TrackBuffersManager::InitializationSegme
 }
 
 void
 TrackBuffersManager::OnDemuxerInitDone(nsresult)
 {
   MOZ_ASSERT(OnTaskQueue());
   mDemuxerInitRequest.Complete();
 
-  // mInputDemuxer shouldn't have been destroyed while a demuxer init/reset
-  // request was being processed. See bug 1239983.
-  MOZ_DIAGNOSTIC_ASSERT(mInputDemuxer);
+  if (!mInputDemuxer) {
+    // mInputDemuxer shouldn't have been destroyed while a demuxer init/reset
+    // request was being processed. See bug 1239983.
+    NS_WARNING("mInputDemuxer has been destroyed");
+    RejectAppend(NS_ERROR_ABORT, __func__);
+  }
 
   MediaInfo info;
 
   uint32_t numVideos = mInputDemuxer->GetNumberTracks(TrackInfo::kVideoTrack);
   if (numVideos) {
     // We currently only handle the first video track.
     mVideoTracks.mDemuxer = mInputDemuxer->GetTrackDemuxer(TrackInfo::kVideoTrack, 0);
     MOZ_ASSERT(mVideoTracks.mDemuxer);