--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -2911,16 +2911,20 @@ public:
                            TrackID aDestinationTrackID)
     : MediaStreamTrackSource(aCapturedTrackSource->GetPrincipal(),
                              nsString())
     , mElement(aElement)
     , mCapturedTrackSource(aCapturedTrackSource)
     , mOwningStream(aOwningStream)
     , mDestinationTrackID(aDestinationTrackID)
   {
+    MOZ_ASSERT(mElement);
+    MOZ_ASSERT(mCapturedTrackSource);
+    MOZ_ASSERT(mOwningStream);
+    MOZ_ASSERT(IsTrackIDExplicit(mDestinationTrackID));
   }
 
   void Destroy() override
   {
     if (mCapturedTrackSource) {
       mCapturedTrackSource->UnregisterSink(this);
       mCapturedTrackSource = nullptr;
     }