Preloader mask (setMask)

I’m building a little preloader with one of those animated barberpole things (regular loadbar but with animated .png moving inside of it)

i’m using a movieclip with a long, skinny .png inside of it for the animation. the .png moves slowly from left to right and loops to give the barber pole effect.

then above that on another layer I have my loadbar mc which is just a green rectangle inside an mc with left regisration point.

everything works great but the loadbar mc is not masking the barberpole mc.

i’m using:

barberPole.setMask(loadBar);

because using the layer as a mask doesn’t seem to work.
Anybody got any ideas???