ActionScript 3.0 :: How To Animate Mask In Runtime

Trying to animate a mask in runtime:

Code:
var _mask = new Sprite();
_mask.graphics.beginFill(0x000000);
_mask.graphics.drawRect(0,0,mc.width,mc.height);
_mask.graphics.endFill();
mc.mask = _mask;
var timer:Timer = new Timer(100);
var foo:Function = function (e:TimerEvent) {
_mask.x += 1;
if (_mask.x > mc.width) timer.stop();
} timer.addEventListener(TimerEvent.TIMER, foo);
timer.start();

Nothing happens... FYI I tried with Tween too but the same result. I know that mask only works when the associated displayobject is on the display list. Can it be something with that? If I set the x of _mask staticly (no animation) it works.