Alpha Mask

[shapper]

Hi,

I created 2 layers. One has an image and the other has a rectangle with a gradient alpha going from 0% (left) to 100%(right).
Then I make this mask of the layer that contains the image.
It works fine in Flash but when I preview the file the mask does not work.

What is going wrong? I am using Flash 9.
I tried everything I could think of ...

Thanks,
Miguel