Tweened gradient mask not working above flash 8

I’m using a gradient mask in Flash CS4 and have it tweened - when I publish it in flash 10, although the gradient is visibly masked - it doesn’t move. When I publish it in flash 8, however, it moves. I’m using the code:

maskMC.cacheAsBitmap = true;
maskee.cacheAsBitmap = true;

maskee.setMask( maskMC );

Can anyone please tell me how to get this to work with flash 9 or 10 publishing?