Problem with masking using shapes

I encountered a strange problem.
I’m trying to mask dynamic text (I embedded the font).
In the mask layer I have a shape that changes position and shape, not with tweens bit with sort of a keyframe animation. However, this won’t work apparently.
When I have an mc in the mask layer, the text does show up as it should, but then I can’t change the shape of the mask.

To give you an idea of what I’m trying to achieve:
http://www.arjengosman.nl/magazine/ (390 kb, no preloader)

ATM I removed everything from the mask layer, so you can see the text is on top of the thumb while it should’nt be visible in that area.

Any ideas why I can’t use ‘keyframed’ shapes as a mask?

Thanks a lot.