Tween Drawing API Rectangle

I thought this was going to be easy, but…

I’m trying to create a rectangular mask - located at x:0, y:173 (1 pixel high ) - that will be stretched (_yscale) to the bottom of the image using the Flash Tween Class. Trouble is, the entire rectangle moves as if if were using “_y” instead of “_yscale”. Tweening “_height” doesn’t work any better. Can someone spot the problem with this file? If you comment out this line

var mask_tween:Object = new Tween(maskClip, “_yscale”, Strong.easeOut, 0, 100, 1, true);

then you will see the correct placement of the mask rectangle. It just doesn’t tween/scale properly.