--- a/dom/media/AudioMixer.h
+++ b/dom/media/AudioMixer.h
@@ -58,21 +58,22 @@ public:
   }
 
   /* Get the data from the mixer. This is supposed to be called when all the
    * tracks have been mixed in. The caller should not hold onto the data. */
   void FinishMixing() {
     MOZ_ASSERT(mChannels && mFrames && mSampleRate, "Mix not called for this cycle?");
     for (MixerCallback* cb = mCallbacks.getFirst();
          cb != nullptr; cb = cb->getNext()) {
-      cb->mReceiver->MixerCallback(mMixedAudio.Elements(),
-                                   AudioSampleTypeToFormat<AudioDataValue>::Format,
-                                   mChannels,
-                                   mFrames,
-                                   mSampleRate);
+      MixerCallbackReceiver* receiver = cb->mReceiver;
+      receiver->MixerCallback(mMixedAudio.Elements(),
+                              AudioSampleTypeToFormat<AudioDataValue>::Format,
+                              mChannels,
+                              mFrames,
+                              mSampleRate);
     }
     PodZero(mMixedAudio.Elements(), mMixedAudio.Length());
     mSampleRate = mChannels = mFrames = 0;
   }
 
   /* Add a buffer to the mix. The buffer can be null if there's nothing to mix
    * but the callback is still needed. */
   void Mix(AudioDataValue* aSamples,