"All target frames are opaque ..."

I'm using CAF on a video, getting rid of a bit of lens dirt. I've made several passes using both AE's auto fill and Reference Frames. I need to do more with this mask (AE isn't doing a great job of healing the selected area) but  I'm getting the popup telling me that All targeted frames are opaque ...

I've restarted AE, clearing preferences as one post recommended - didn't change things. I've made sure that the mask is set to Subtract, so that's not a problem. Does AE allow only so much alteration to a masked clip? Do I need to make a new mask that runs side-by-side, or on top of, the first mask?

Thanks for any help you can provide.