diff --git a/dom/media/MediaDecoderReaderWrapper.cpp b/dom/media/MediaDecoderReaderWrapper.cpp
index e5404aac2bd3..137910a18386 100644
--- a/dom/media/MediaDecoderReaderWrapper.cpp
+++ b/dom/media/MediaDecoderReaderWrapper.cpp
@@ -13,7 +13,11 @@ MediaDecoderReaderWrapper::MediaDecoderReaderWrapper(AbstractThread* aOwnerThrea
                                                      MediaDecoderReader* aReader)
   : mOwnerThread(aOwnerThread)
   , mReader(aReader)
-{}
+{
+  // Must support either heuristic buffering or WaitForData().
+  MOZ_ASSERT(mReader->UseBufferingHeuristics() ||
+             mReader->IsWaitForDataSupported());
+}
 
 MediaDecoderReaderWrapper::~MediaDecoderReaderWrapper()
 {}
@@ -92,6 +96,7 @@ RefPtr<MediaDecoderReaderWrapper::WaitForDataPromise>
 MediaDecoderReaderWrapper::WaitForData(MediaData::Type aType)
 {
   MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+  MOZ_ASSERT(mReader->IsWaitForDataSupported());
   return InvokeAsync(mReader->OwnerThread(), mReader.get(), __func__,
                      &MediaDecoderReader::WaitForData, aType);
 }
diff --git a/dom/media/MediaDecoderReaderWrapper.h b/dom/media/MediaDecoderReaderWrapper.h
index e9723ceacd48..01c9bd2edb50 100644
--- a/dom/media/MediaDecoderReaderWrapper.h
+++ b/dom/media/MediaDecoderReaderWrapper.h
@@ -52,7 +52,6 @@ public:
   void ResetDecode(TrackSet aTracks);
 
   nsresult Init() { return mReader->Init(); }
-  bool IsWaitForDataSupported() const { return mReader->IsWaitForDataSupported(); }
   bool IsAsync() const { return mReader->IsAsync(); }
   bool UseBufferingHeuristics() const { return mReader->UseBufferingHeuristics(); }
 
diff --git a/dom/media/MediaDecoderStateMachine.cpp b/dom/media/MediaDecoderStateMachine.cpp
index 6c4453f5950e..cbbaac5e667d 100644
--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -236,8 +236,8 @@ protected:
     // 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() &&
-            (mMaster->IsWaitingAudioData() || mMaster->IsWaitingVideoData()));
+           mMaster->IsWaitingAudioData() ||
+           mMaster->IsWaitingVideoData();
   }
   MediaQueue<MediaData>& AudioQueue() const { return mMaster->mAudioQueue; }
   MediaQueue<MediaData>& VideoQueue() const { return mMaster->mVideoQueue; }
@@ -1746,8 +1746,6 @@ StateObject::HandleNotDecoded(MediaData::Type aType, const MediaResult& aError)
   // If the decoder is waiting for data, we tell it to call us back when the
   // data arrives.
   if (aError == NS_ERROR_DOM_MEDIA_WAITING_FOR_DATA) {
-    MOZ_ASSERT(Reader()->IsWaitForDataSupported(),
-               "Readers that send WAITING_FOR_DATA need to implement WaitForData");
     mMaster->WaitForData(aType);
     HandleWaitingForData();
     return;
@@ -2077,7 +2075,6 @@ DecodingState::MaybeStartBuffering()
                    mMaster->HasLowDecodedData() &&
                    mMaster->HasLowBufferedData();
   } else {
-    MOZ_ASSERT(Reader()->IsWaitForDataSupported());
     shouldBuffer =
       (mMaster->OutOfDecodedAudio() && mMaster->IsWaitingAudioData()) ||
       (mMaster->OutOfDecodedVideo() && mMaster->IsWaitingVideoData());
@@ -2165,8 +2162,6 @@ BufferingState::Step()
       return;
     }
   } else if (mMaster->OutOfDecodedAudio() || mMaster->OutOfDecodedVideo()) {
-    MOZ_ASSERT(Reader()->IsWaitForDataSupported(),
-               "Don't yet have a strategy for non-heuristic + non-WaitForData");
     mMaster->DispatchDecodeTasksIfNeeded();
     MOZ_ASSERT(mMaster->mMinimizePreroll ||
                !mMaster->OutOfDecodedAudio() ||