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!