--- a/dom/animation/DocumentTimeline.cpp
+++ b/dom/animation/DocumentTimeline.cpp
@@ -128,16 +128,19 @@ DocumentTimeline::ToTimelineTime(const T
                   - timing->GetNavigationStartTimeStamp()
                   - mOriginTime);
   return result;
 }
 
 void
 DocumentTimeline::NotifyAnimationUpdated(Animation& aAnimation)
 {
+  MOZ_ASSERT(aAnimation.GetTimeline() == this,
+             "Should be used for animations that the animation's timeline "
+             "is this DocumentTimeline");
   MOZ_ASSERT(aAnimation.GetTimelineDocument() &&
                aAnimation.GetTimelineDocument() == mDocument,
              "Should be used for animations that the animation's timeline "
              "document is the same document of this DocumentTimeline");
 
   AnimationTimeline::NotifyAnimationUpdated(aAnimation);
 
   if (!mIsObservingRefreshDriver) {