AttachMovie-LoadMovie-setMask...help

I have a movieclip (“thumb”) whith 2 layers. In the top layer a movieclip to be used as mask (“foto_mascara”) of the bottom layer movieclip (“foto_contenedor”) and try this:

_root.attachMovie(“thumb”,“thumb2”,_root.getNextHighestDepth ());
nueva_foto=eval(“thumb2”);
loadMovie(fotos*,nueva_foto.foto_contenedor);
nueva_foto.foto_contenedor.setMask(nueva_foto.foto_mascara);

but the problem is that the “foto_contenedor” movieclip shows the entire image under the “foto_mascara” movieclip.

And if i do this ,the mask works right:

_root.attachMovie(“thumb”,“thumb2”,_root.getNextHighestDepth ());
nueva_foto=eval(“thumb2”);
nueva_foto.foto_contenedor.createEmptyMovieClip(“foto”,nueva_foto.foto_contenedor.getNextHighestDepth());
loadMovie(fotos*,nueva_foto.foto_contenedor.foto);
nueva_foto.foto_contenedor.setMask(nueva_foto.foto_mascara);

is that the way to do it?..creating an empty movieclip inside the container?..