Dynamic Mask Problem

I am creating two empty movie clips on the stage and using a MovieClipLoader to load content into them and that works just fine. I have a mask that I am placing two instances of on the stage and assigning the masks to the two clips (one each of course).

The mask works great, but when I call functions that move the emptyClips i have to reassign the masks and there is a little flash where the mask become visible for one frame. Does anyone know how to prevent this? :ninja: