--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -2539,33 +2539,28 @@ HTMLMediaElement::AddCaptureMediaTrackTo
        track.get(), destinationTrackID, inputTrack, port.get()));
 }
 
 already_AddRefed<DOMMediaStream>
 HTMLMediaElement::CaptureStreamInternal(bool aFinishWhenEnded,
                                         bool aCaptureAudio,
                                         MediaStreamGraph* aGraph)
 {
+  MOZ_RELEASE_ASSERT(aGraph);
+
   nsPIDOMWindowInner* window = OwnerDoc()->GetInnerWindow();
   if (!window) {
     return nullptr;
   }
 #ifdef MOZ_EME
   if (ContainsRestrictedContent()) {
     return nullptr;
   }
 #endif
 
-  if (!aGraph) {
-    MediaStreamGraph::GraphDriverType graphDriverType =
-      HasAudio() ? MediaStreamGraph::AUDIO_THREAD_DRIVER
-                 : MediaStreamGraph::SYSTEM_THREAD_DRIVER;
-    aGraph = MediaStreamGraph::GetInstance(graphDriverType, mAudioChannel);
-  }
-
   if (!mOutputStreams.IsEmpty() &&
       aGraph != mOutputStreams[0].mStream->GetInputStream()->Graph()) {
     return nullptr;
   }
 
   OutputMediaStream* out = mOutputStreams.AppendElement();
   MediaStreamTrackSourceGetter* getter = new CaptureStreamTrackSourceGetter(this);
   out->mStream = DOMMediaStream::CreateTrackUnionStreamAsInput(window, aGraph, getter);
@@ -2650,43 +2645,58 @@ HTMLMediaElement::CaptureStreamInternal(
   RefPtr<DOMMediaStream> result = out->mStream;
   return result.forget();
 }
 
 already_AddRefed<DOMMediaStream>
 HTMLMediaElement::CaptureAudio(ErrorResult& aRv,
                                MediaStreamGraph* aGraph)
 {
-  RefPtr<DOMMediaStream> stream = CaptureStreamInternal(false, aGraph);
+  MOZ_RELEASE_ASSERT(aGraph);
+
+  RefPtr<DOMMediaStream> stream =
+    CaptureStreamInternal(false, true, aGraph);
   if (!stream) {
     aRv.Throw(NS_ERROR_FAILURE);
     return nullptr;
   }
 
   return stream.forget();
 }
 
 already_AddRefed<DOMMediaStream>
-HTMLMediaElement::MozCaptureStream(ErrorResult& aRv,
-                                   MediaStreamGraph* aGraph)
-{
-  RefPtr<DOMMediaStream> stream = CaptureStreamInternal(false, aGraph);
+HTMLMediaElement::MozCaptureStream(ErrorResult& aRv)
+{
+  MediaStreamGraph::GraphDriverType graphDriverType =
+    HasAudio() ? MediaStreamGraph::AUDIO_THREAD_DRIVER
+               : MediaStreamGraph::SYSTEM_THREAD_DRIVER;
+  MediaStreamGraph* graph =
+    MediaStreamGraph::GetInstance(graphDriverType, mAudioChannel);
+
+  RefPtr<DOMMediaStream> stream =
+    CaptureStreamInternal(false, false, graph);
   if (!stream) {
     aRv.Throw(NS_ERROR_FAILURE);
     return nullptr;
   }
 
   return stream.forget();
 }
 
 already_AddRefed<DOMMediaStream>
-HTMLMediaElement::MozCaptureStreamUntilEnded(ErrorResult& aRv,
-                                             MediaStreamGraph* aGraph)
-{
-  RefPtr<DOMMediaStream> stream = CaptureStreamInternal(true, aGraph);
+HTMLMediaElement::MozCaptureStreamUntilEnded(ErrorResult& aRv)
+{
+  MediaStreamGraph::GraphDriverType graphDriverType =
+    HasAudio() ? MediaStreamGraph::AUDIO_THREAD_DRIVER
+               : MediaStreamGraph::SYSTEM_THREAD_DRIVER;
+  MediaStreamGraph* graph =
+    MediaStreamGraph::GetInstance(graphDriverType, mAudioChannel);
+
+  RefPtr<DOMMediaStream> stream =
+    CaptureStreamInternal(true, false, graph);
   if (!stream) {
     aRv.Throw(NS_ERROR_FAILURE);
     return nullptr;
   }
 
   return stream.forget();
 }
 
@@ -6560,17 +6570,18 @@ HTMLMediaElement::AudioCaptureStreamChan
     uint64_t id = window->WindowID();
     MediaStreamGraph* msg =
       MediaStreamGraph::GetInstance(MediaStreamGraph::AUDIO_THREAD_DRIVER,
                                     mAudioChannel);
 
     if (GetSrcMediaStream()) {
       mCaptureStreamPort = msg->ConnectToCaptureStream(id, GetSrcMediaStream());
     } else {
-      RefPtr<DOMMediaStream> stream = CaptureStreamInternal(false, msg);
+      RefPtr<DOMMediaStream> stream =
+        CaptureStreamInternal(false, false, msg);
       mCaptureStreamPort = msg->ConnectToCaptureStream(id, stream->GetPlaybackStream());
     }
   } else if (!mAudioCapturedByWindow && mCaptureStreamPort) {
     if (mDecoder) {
       ProcessedMediaStream* ps =
         mCaptureStreamPort->GetSource()->AsProcessedStream();
       MOZ_ASSERT(ps);
 

--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -650,23 +650,21 @@ public:
   void CannotDecryptWaitingForKey();
 
   bool MozAutoplayEnabled() const
   {
     return mAutoplayEnabled;
   }
 
   already_AddRefed<DOMMediaStream> CaptureAudio(ErrorResult& aRv,
-                                                MediaStreamGraph* aGraph = nullptr);
+                                                MediaStreamGraph* aGraph);
 
-  already_AddRefed<DOMMediaStream> MozCaptureStream(ErrorResult& aRv,
-                                                    MediaStreamGraph* aGraph = nullptr);
+  already_AddRefed<DOMMediaStream> MozCaptureStream(ErrorResult& aRv);
 
-  already_AddRefed<DOMMediaStream> MozCaptureStreamUntilEnded(ErrorResult& aRv,
-                                                              MediaStreamGraph* aGraph = nullptr);
+  already_AddRefed<DOMMediaStream> MozCaptureStreamUntilEnded(ErrorResult& aRv);
 
   bool MozAudioCaptured() const
   {
     return mAudioCaptured;
   }
 
   void MozGetMetadata(JSContext* aCx, JS::MutableHandle<JSObject*> aResult,
                       ErrorResult& aRv);
@@ -892,17 +890,17 @@ protected:
    * The stream will never finish.
    *
    * When aCaptureAudio is true, we stop playout of audio and instead route it
    * to the DOMMediaStream. Volume and mute state will be applied to the audio
    * reaching the stream. No video tracks will be captured in this case.
    */
   already_AddRefed<DOMMediaStream> CaptureStreamInternal(bool aFinishWhenEnded,
                                                          bool aCaptureAudio,
-                                                         MediaStreamGraph* aGraph = nullptr);
+                                                         MediaStreamGraph* aGraph);
 
   /**
    * Initialize a decoder as a clone of an existing decoder in another
    * element.
    * mLoadingSrc must already be set.
    */
   nsresult InitializeDecoderAsClone(MediaDecoder* aOriginal);