Simple animated gradient mask?

Hi,

Thought i knew this one, a simple animated mask with feather alpha.

I have just a green image and a mask animated from left to right. The mask has a gradient fill.
Now to get the gradient mask effect i thought i just had to add the following AS

gradientMask.cacheAsBitmap = true;
green_img_mc.cacheAsBitmap = true;

green_img_mc.setMask(gradientMask);

but now the mask isn’t moving.

How can i make this simple effect to work?

regards

ps: if i set publish to AS3 and i use green_img_mc.mask =gradientMask; it works but i need a AS2 solution

ps2: publishing for flash player 8 my mask is also movinge (src: http://www.kirupa.com/forum/showthread.php?335080-Tweened-gradient-mask-not-working-above-flash-8)