How to display section in timeline without frames

I've an app, where the user can play an animation. The animation progress is indicated in the timeline (with blue color). The playable, but not active part of the timeline has gray color. The timeline has also part, where are no frames, but I need to somehow display this part of the timeline too. And this is the tricky part. Currently I have displayed this section with border, but it's looking strange. I've also tried not displaying it at all, but my lead returned it, because for the user is useful to know that there aren't frames, and it's part of the timeline.