setMask with mc Tween problem

This doesnt quite work like I want. The setMask propery it not masking an externally loaded clip.

Basically the mask_mc should mask any movie loaded into holder_mc. The user will click on a button which should tell the holder_mc what movie to load and should shrink the mask_mc. After the movie is loaded, the mask_mc should grow to the specified dimensions, showing the new movie. In theory this should be pretty simple, but it just doesnt seem to work.

This is a perfect little movie for any type of site. It could have also more funtionality with the mask tween, but I’m not shure yet how to set that.