--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -823,24 +823,19 @@ MediaDecoderStateMachine::IsVideoDecodin
   MOZ_ASSERT(OnTaskQueue());
   return HasVideo() && !VideoQueue().IsFinished();
 }
 
 void
 MediaDecoderStateMachine::CheckIfDecodeComplete()
 {
   MOZ_ASSERT(OnTaskQueue());
-
-  if (IsShutdown() ||
-      mState == DECODER_STATE_SEEKING ||
-      mState == DECODER_STATE_COMPLETED) {
-    // Don't change our state if we've already been shutdown, or we're seeking,
-    // since we don't want to abort the shutdown or seek processes.
-    return;
-  }
+  MOZ_ASSERT(mState == DECODER_STATE_DECODING ||
+             mState == DECODER_STATE_BUFFERING);
+
   if (!IsVideoDecoding() && !IsAudioDecoding()) {
     // We've finished decoding all active streams,
     // so move to COMPLETED state.
     SetState(DECODER_STATE_COMPLETED);
     DispatchDecodeTasksIfNeeded();
     ScheduleStateMachine();
   }
   DECODER_LOG("CheckIfDecodeComplete %scompleted",