--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -2418,17 +2418,17 @@ MediaCacheStream::Unpin()
   // Queue an Update since we may be able to read more into the
   // cache, if this stream's block have become evictable
   mMediaCache->QueueUpdate(lock);
 }
 
 int64_t
 MediaCacheStream::GetLength()
 {
-  // TODO: Assert non-main thread.
+  MOZ_ASSERT(!NS_IsMainThread());
   AutoLock lock(mMediaCache->Monitor());
   return mStreamLength;
 }
 
 int64_t
 MediaCacheStream::GetNextCachedData(int64_t aOffset)
 {
   MOZ_ASSERT(!NS_IsMainThread());