Help with Set Matte

Hey, I want to make sure the pupil of the eye I made doesn't phase out of the white of the eye. I tried using Set Matte and linking the pupil to the white of the eye. In all of the tutorials I've followed I've noticed that all of the images they used are shapes so I thought maybe that was the problem? I did that with the white part but it still doesn't work, does that even matter? Any help would be greatly appreciated!!