--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -229,16 +229,19 @@ public:
   class ResourceStreamIterator
   {
   public:
     ResourceStreamIterator(MediaCache* aMediaCache, int64_t aResourceID)
       : mMediaCache(aMediaCache)
       , mResourceID(aResourceID)
       , mNext(0)
     {
+      MOZ_ASSERT(
+        NS_IsMainThread() ||
+        (aMediaCache->GetReentrantMonitor().AssertCurrentThreadIn(), true));
     }
     MediaCacheStream* Next()
     {
       while (mNext < mMediaCache->mStreams.Length()) {
         MediaCacheStream* stream = mMediaCache->mStreams[mNext];
         ++mNext;
         if (stream->GetResourceID() == mResourceID && !stream->IsClosed())
           return stream;