Circle turns into polygon

Hey guys,

Need some help here…
Probably a quick one…

I have this rectangle centered on the stage.
I made a circle mask tween above the rectangle. 1X1 pixel on first frame to big enough to show the whole rectangle on the last frame.

The thing is… When i play the animation by pressing Enter while on the workspace. It looks perfectly ok.

But when i preview it using text movie or with the exported movie. the circle mask seems to be a polygon.

Heres a print screen of the mask animation probably halfway through…

Why is this happening?..

Anybody with an explanation and/or solution about this?

Thanks is advance.