White outline when circle is on top of tween

I have a circle on top of an image that's fading in and out.  When I tween the image, a white border appears around the circles on top of the image.  This doesn't happen when the circle is below the image layer, and it doesn't happen with a square.  Any idea what's going on?

Below is an image of the hairline white border around the black and blue circle.

I'm not sure if it matters, but here is my AS:

import fl.transitions.Tween;
import fl.transitions.TweenEvent;
import fl.transitions.easing.*;
import flash.events.*;

var curTween:Tween = new Tween(testImg, "alpha", Strong.easeOut, 100, 0, 3, true);
curTween.addEventListener(TweenEvent.MOTION_FINISH, fadeBackIn);

function fadeBackIn(event:TweenEvent) {
     var nxtTween:Tween = new Tween(testImg, "alpha", Strong.easeIn, 0, 100, 3, true);
}