--- a/dom/media/MediaDecoderStateMachine.h
+++ b/dom/media/MediaDecoderStateMachine.h
@@ -898,17 +898,17 @@ private:
   // Media Fragment end time in microseconds. Access controlled by decoder monitor.
   int64_t mFragmentEndTime;
 
   // The media sink resource.  Used on the state machine thread.
   RefPtr<media::MediaSink> mMediaSink;
 
   // The reader, don't call its methods with the decoder monitor held.
   // This is created in the state machine's constructor.
-  RefPtr<MediaDecoderReader> mReader;
+  const RefPtr<MediaDecoderReader> mReader;
 
   // The end time of the last audio frame that's been pushed onto the media sink
   // in microseconds. This will approximately be the end time
   // of the audio stream, unless another frame is pushed to the hardware.
   int64_t AudioEndTime() const;
 
   // The end time of the last rendered video frame that's been sent to
   // compositor.