--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -1071,16 +1071,19 @@ MediaDecoderStateMachine::SetState(State
 void
 MediaDecoderStateMachine::ExitState()
 {
   MOZ_ASSERT(OnTaskQueue());
   switch (mState) {
     case DECODER_STATE_COMPLETED:
       mSentPlaybackEndedEvent = false;
       break;
+    case DECODER_STATE_SHUTDOWN:
+      MOZ_DIAGNOSTIC_ASSERT(false, "Shouldn't escape the SHUTDOWN state.");
+      break;
     default:
       break;
   }
 }
 
 void
 MediaDecoderStateMachine::EnterState()
 {