--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -225,16 +225,24 @@ public:
 
 protected:
   using Master = MediaDecoderStateMachine;
   explicit StateObject(Master* aPtr) : mMaster(aPtr) {}
   TaskQueue* OwnerThread() const { return mMaster->mTaskQueue; }
   MediaResource* Resource() const { return mMaster->mResource; }
   MediaDecoderReaderWrapper* Reader() const { return mMaster->mReader; }
   const MediaInfo& Info() const { return mMaster->Info(); }
+  bool IsExpectingMoreData() const
+  {
+    // We are expecting more data if either the resource states so, or if we
+    // have a waiting promise pending (such as with non-MSE EME).
+    return Resource()->IsExpectingMoreData() ||
+           (Reader()->IsWaitForDataSupported() &&
+            (Reader()->IsWaitingAudioData() || Reader()->IsWaitingVideoData()));
+  }
 
   // Note this function will delete the current state object.
   // Don't access members to avoid UAF after this call.
   template <class S, typename... Ts>
   auto SetState(Ts... aArgs)
     -> decltype(DeclVal<S>().Enter(Move(aArgs)...))
   {
     // keep mMaster in a local object because mMaster will become invalid after
@@ -1466,17 +1474,17 @@ DecodingState::MaybeStartBuffering()
   // Don't enter buffering while prerolling so that the decoder has a chance to
   // enqueue some decoded data before we give up and start buffering.
   if (!mMaster->IsPlaying()) {
     return;
   }
 
   bool shouldBuffer;
   if (Reader()->UseBufferingHeuristics()) {
-    shouldBuffer = Resource()->IsExpectingMoreData() &&
+    shouldBuffer = IsExpectingMoreData() &&
                    mMaster->HasLowDecodedData() &&
                    mMaster->HasLowBufferedData();
   } else {
     MOZ_ASSERT(Reader()->IsWaitForDataSupported());
     shouldBuffer =
       (mMaster->OutOfDecodedAudio() && Reader()->IsWaitingAudioData()) ||
       (mMaster->OutOfDecodedVideo() && Reader()->IsWaitingVideoData());
   }
@@ -1606,17 +1614,17 @@ BufferingState::Step()
   // we've not decoded enough data to begin playback, or if we've not
   // downloaded a reasonable amount of data inside our buffering time.
   if (Reader()->UseBufferingHeuristics()) {
     TimeDuration elapsed = now - mBufferingStart;
     bool isLiveStream = Resource()->IsLiveStream();
     if ((isLiveStream || !mMaster->CanPlayThrough()) &&
         elapsed < TimeDuration::FromSeconds(mBufferingWait * mMaster->mPlaybackRate) &&
         mMaster->HasLowBufferedData(mBufferingWait * USECS_PER_S) &&
-        Resource()->IsExpectingMoreData()) {
+        IsExpectingMoreData()) {
       SLOG("Buffering: wait %ds, timeout in %.3lfs",
            mBufferingWait, mBufferingWait - elapsed.ToSeconds());
       mMaster->ScheduleStateMachineIn(USECS_PER_S);
       return;
     }
   } else if (mMaster->OutOfDecodedAudio() || mMaster->OutOfDecodedVideo()) {
     MOZ_ASSERT(Reader()->IsWaitForDataSupported(),
                "Don't yet have a strategy for non-heuristic + non-WaitForData");