Masks not working, producing strange results?...

Hello, I'm having problems with an animated mask.

I have a leg graphic (contains 3 tweened leg pieces), and then a gradient.

In the editor they behave how they should, properly masking the leg. However

the SWF output produces some crazy results shown below:

I've tried setting both as movieclips, using the "Layer" blend mode on the leg, using leg.mask = legMask;

but whatever I try produces the exact same result.

Any ideas?

Thanks!