diff --git a/dom/media/DOMMediaStream.cpp b/dom/media/DOMMediaStream.cpp
index a42f08553c9f..f46cf6e14ab7 100644
--- a/dom/media/DOMMediaStream.cpp
+++ b/dom/media/DOMMediaStream.cpp
@@ -711,7 +711,7 @@ DOMMediaStream::CloneInternal(TrackForwardingOption aForwarding)
 }
 
 MediaStreamTrack*
-DOMMediaStream::GetTrackById(const nsString& aId)
+DOMMediaStream::GetTrackById(const nsAString& aId) const
 {
   for (const RefPtr<TrackPort>& info : mTracks) {
     nsString id;
@@ -724,7 +724,7 @@ DOMMediaStream::GetTrackById(const nsString& aId)
 }
 
 MediaStreamTrack*
-DOMMediaStream::GetOwnedTrackById(const nsString& aId)
+DOMMediaStream::GetOwnedTrackById(const nsAString& aId)
 {
   for (const RefPtr<TrackPort>& info : mOwnedTracks) {
     nsString id;
diff --git a/dom/media/DOMMediaStream.h b/dom/media/DOMMediaStream.h
index 2ebc7361855f..2920897bcea4 100644
--- a/dom/media/DOMMediaStream.h
+++ b/dom/media/DOMMediaStream.h
@@ -352,6 +352,7 @@ public:
   void GetAudioTracks(nsTArray<RefPtr<AudioStreamTrack> >& aTracks) const;
   void GetVideoTracks(nsTArray<RefPtr<VideoStreamTrack> >& aTracks) const;
   void GetTracks(nsTArray<RefPtr<MediaStreamTrack> >& aTracks) const;
+  MediaStreamTrack* GetTrackById(const nsAString& aId) const;
   void AddTrack(MediaStreamTrack& aTrack);
   void RemoveTrack(MediaStreamTrack& aTrack);
 
@@ -374,9 +375,7 @@ public:
   };
   already_AddRefed<DOMMediaStream> CloneInternal(TrackForwardingOption aForwarding);
 
-  MediaStreamTrack* GetTrackById(const nsString& aId);
-
-  MediaStreamTrack* GetOwnedTrackById(const nsString& aId);
+  MediaStreamTrack* GetOwnedTrackById(const nsAString& aId);
 
   /**
    * Returns true if this DOMMediaStream has aTrack in its mPlaybackStream.
diff --git a/dom/webidl/MediaStream.webidl b/dom/webidl/MediaStream.webidl
index 60f71022d0e7..5b8a42a246c0 100644
--- a/dom/webidl/MediaStream.webidl
+++ b/dom/webidl/MediaStream.webidl
@@ -36,7 +36,7 @@ interface MediaStream : EventTarget {
     sequence<AudioStreamTrack> getAudioTracks ();
     sequence<VideoStreamTrack> getVideoTracks ();
     sequence<MediaStreamTrack> getTracks ();
-    // MediaStreamTrack?          getTrackById (DOMString trackId);
+    MediaStreamTrack?          getTrackById (DOMString trackId);
     void                       addTrack (MediaStreamTrack track);
     void                       removeTrack (MediaStreamTrack track);
     MediaStream                clone ();