--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -1670,19 +1670,17 @@ MediaDecoder::GetStateMachine() const {
   MOZ_ASSERT(NS_IsMainThread());
   return mDecoderStateMachine;
 }
 
 void
 MediaDecoder::FireTimeUpdate()
 {
   MOZ_ASSERT(NS_IsMainThread());
-  if (IsShutdown()) {
-    return;
-  }
+  MOZ_ASSERT(!IsShutdown());
   mOwner->FireTimeUpdate(true);
 }
 
 void
 MediaDecoder::PinForSeek()
 {
   MOZ_ASSERT(NS_IsMainThread());
   MediaResource* resource = GetResource();