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Set Mask to a movieClip with complex shape


I want to create a ripple effect. I need a ring mask, so I make a movieClip containing a ring. In that movieClip, I draw a small circle among a large circle then delete the inner small circle. It looks like a ring.

However, when I setMask to this movieClip, I dont get a ring mask ! Why ?

Here is a movie done by Flash MX. A grid movieClip setMask to a rotating movieClip. The rotating mask movieClip consists of a square with a center circular hole.

How do we create such movieClip ?

Simple. Create a new movieClip symbol, draw a filled square. Then draw a circle in the center of the square. Select the circle and remove the circle. After this, drag it to the grid and make rotation and setMask.

If you try it, possibly, you will get a movie like below. It looks like the central circle is not removed.

You can try to create more complex mask by combination of several shape. Just pick a square and a circle in different combination. We will get unexpected result.

This weird effect will not appear if you create mask layer instead of script it setMask;

To solve this, I need to select all the shape and fill it with the same color, then  expand 1 pixle and inset 1 pixel. Then it works.