Scaling causing rendering flaws?

If I draw a contiguous grid of solid black rectangles in a sprite, say 4 high and 5 wide, and then I tween the scalex and scaley of the containing sprite equally over time, at certain scale values the rectangles draw incorrectly such that you can see gaps between the rectangles which results in ugly flashing of lines between the grid rectangles as the tween occurs. If they are there at the stop point of the tween it’s particularly annoying.

(My problem is actually with cutting up an image into a contiguous grid and seeing these lines when scaling, but an all black grid is a simple way to demonstrate the situation.)

I’m assuming some optimization in Flash Player’s scaling math is causing this.

Does anyone know if there is a way to avoid this? I assume if there is it will be via math - maybe by preparing the widths so they are always even or something? I tried ensuring the dimensions were always integers but that didn’t fix it.