--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -243,17 +243,21 @@ public:
 
 #ifdef DEBUG_VERIFY_CACHE
   // Verify invariants, especially block list invariants
   void Verify(AutoLock&);
 #else
   void Verify(AutoLock&) {}
 #endif
 
-  ReentrantMonitor& Monitor() { return mMonitor; }
+  ReentrantMonitor& Monitor()
+  {
+    MOZ_DIAGNOSTIC_ASSERT(!NS_IsMainThread());
+    return mMonitor;
+  }
 
   /**
    * An iterator that makes it easy to iterate through all streams that
    * have a given resource ID and are not closed.
    * Must be used while holding the media cache lock.
    */
   class ResourceStreamIterator
   {