diff --git a/content/media/MediaSegment.h b/content/media/MediaSegment.h
index b608ca758c36..be94cfb174f4 100644
--- a/content/media/MediaSegment.h
+++ b/content/media/MediaSegment.h
@@ -100,6 +100,10 @@ public:
    * Insert aDuration of null data at the end of the segment.
    */
   virtual void AppendNullData(TrackTicks aDuration) = 0;
+  /**
+   * Remove all contents, setting duration to 0.
+   */
+  virtual void Clear() = 0;
 
 protected:
   MediaSegment(Type aType) : mDuration(0), mType(aType)
@@ -186,6 +190,11 @@ public:
     }
     mDuration += aDuration;
   }
+  virtual void Clear()
+  {
+    mDuration = 0;
+    mChunks.Clear();
+  }
 
   class ChunkIterator {
   public:
diff --git a/content/media/MediaStreamGraph.cpp b/content/media/MediaStreamGraph.cpp
index a061affb4dc4..ba2370c05096 100644
--- a/content/media/MediaStreamGraph.cpp
+++ b/content/media/MediaStreamGraph.cpp
@@ -118,6 +118,9 @@ MediaStreamGraphImpl::ExtractPendingInput(SourceMediaStream* aStream,
       StreamTime t =
         GraphTimeToStreamTime(aStream, mStateComputedTime) +
         (aDesiredUpToTime - mStateComputedTime);
+      LOG(PR_LOG_DEBUG, ("Calling NotifyPull aStream=%p t=%f current end=%f", aStream,
+                         MediaTimeToSeconds(t),
+                         MediaTimeToSeconds(aStream->mBuffer.GetEnd())));
       if (t > aStream->mBuffer.GetEnd()) {
         *aEnsureNextIteration = true;
         for (uint32_t j = 0; j < aStream->mListeners.Length(); ++j) {
@@ -1637,22 +1640,25 @@ SourceMediaStream::AddTrack(TrackID aID, TrackRate aRate, TrackTicks aStart,
   }
 }
 
-void
+bool
 SourceMediaStream::AppendToTrack(TrackID aID, MediaSegment* aSegment)
 {
   MutexAutoLock lock(mMutex);
   // ::EndAllTrackAndFinished() can end these before the sources notice
+  bool appended = false;
   if (!mFinished) {
     TrackData *track = FindDataForTrack(aID);
     if (track) {
       track->mData->AppendFrom(aSegment);
+      appended = true;
     } else {
-      NS_ERROR("Append to non-existent track!");
+      aSegment->Clear();
     }
   }
   if (!mDestroyed) {
     GraphImpl()->EnsureNextIteration();
   }
+  return appended;
 }
 
 bool
@@ -1663,8 +1669,7 @@ SourceMediaStream::HaveEnoughBuffered(TrackID aID)
   if (track) {
     return track->mHaveEnough;
   }
-  NS_ERROR("No track in HaveEnoughBuffered!");
-  return true;
+  return false;
 }
 
 void
@@ -1674,7 +1679,7 @@ SourceMediaStream::DispatchWhenNotEnoughBuffered(TrackID aID,
   MutexAutoLock lock(mMutex);
   TrackData* data = FindDataForTrack(aID);
   if (!data) {
-    NS_ERROR("No track in DispatchWhenNotEnoughBuffered");
+    aSignalThread->Dispatch(aSignalRunnable, 0);
     return;
   }
 
@@ -1694,8 +1699,6 @@ SourceMediaStream::EndTrack(TrackID aID)
     TrackData *track = FindDataForTrack(aID);
     if (track) {
       track->mCommands |= TRACK_END;
-    } else {
-      NS_ERROR("End of non-existant track");
     }
   }
   if (!mDestroyed) {
diff --git a/content/media/MediaStreamGraph.h b/content/media/MediaStreamGraph.h
index 7ecf1a4668a3..025405e51574 100644
--- a/content/media/MediaStreamGraph.h
+++ b/content/media/MediaStreamGraph.h
@@ -567,22 +567,27 @@ public:
   /**
    * Append media data to a track. Ownership of aSegment remains with the caller,
    * but aSegment is emptied.
+   * Returns false if the data was not appended because no such track exists
+   * or the stream was already finished.
    */
-  void AppendToTrack(TrackID aID, MediaSegment* aSegment);
+  bool AppendToTrack(TrackID aID, MediaSegment* aSegment);
   /**
    * Returns true if the buffer currently has enough data.
+   * Returns false if there isn't enough data or if no such track exists.
    */
   bool HaveEnoughBuffered(TrackID aID);
   /**
    * Ensures that aSignalRunnable will be dispatched to aSignalThread
    * when we don't have enough buffered data in the track (which could be
-   * immediately).
+   * immediately). Will dispatch the runnable immediately if the track
+   * does not exist.
    */
   void DispatchWhenNotEnoughBuffered(TrackID aID,
       nsIThread* aSignalThread, nsIRunnable* aSignalRunnable);
   /**
    * Indicate that a track has ended. Do not do any more API calls
    * affecting this track.
+   * Ignored if the track does not exist.
    */
   void EndTrack(TrackID aID);
   /**
@@ -653,7 +658,6 @@ protected:
         return &mUpdateTracks[i];
       }
     }
-    NS_ERROR("Bad track ID!");
     return nullptr;
   }
 
diff --git a/content/media/webrtc/MediaEngineWebRTCVideo.cpp b/content/media/webrtc/MediaEngineWebRTCVideo.cpp
index 200def709386..ba0bb08fdaf9 100644
--- a/content/media/webrtc/MediaEngineWebRTCVideo.cpp
+++ b/content/media/webrtc/MediaEngineWebRTCVideo.cpp
@@ -124,8 +124,11 @@ MediaEngineWebRTCVideoSource::NotifyPull(MediaStreamGraph* aGraph,
   if (delta > 0) {
     // NULL images are allowed
     segment.AppendFrame(image ? image.forget() : nullptr, delta, gfxIntSize(mWidth, mHeight));
-    aSource->AppendToTrack(aID, &(segment));
-    aLastEndTime = target;
+    // This can fail if either a) we haven't added the track yet, or b)
+    // we've removed or finished the track.
+    if (aSource->AppendToTrack(aID, &(segment))) {
+      aLastEndTime = target;
+    }
   }
 }