UI Mask not working

I have a canvas in worldspace. this is the parent for:

The viewport has a scroll rect, an image, and a mask. I do not want to use an image. I just want the rectangle of the viewport to be masked. so on the mask i have (although I have tried using an image but it does not work).

Totally stumped. I have read post after post, tried everything I can think of.

Help! My whole project depends on worldspace UIs and this masking effect!

Go into your project folder and delete the library folder, then restart Unity. That worked for me.