Dynamic masking using the drawing API

Question

I can't quite get this circular mask to work.

onClipEvent(load)
{
        _root.stop();
        dO = 3.6;
        r = 75;
        function addSlice(O)
        {
                x1 = r*Math.sin(O*Math.PI/180);
                x2 = r*Math.sin((O+dO)*Math.PI/180);
                y1 = r*Math.cos((O)*Math.PI/180);
                y2 = r*Math.cos((O+dO)*Math.PI/180);
                trace(x1 + ":" + y1);
                with(_root.mask)
                {
                        moveTo(0,0);
                        beginFill(0x000088);
                        lineTo(x1,y1);
                        lineTo(x2,y2);
                        endFill();
                }
        }
        _root.createEmptyMovieClip("mask",0);
        this.setMask(_root.mask);
        _root.mask._yscale = -100;
        _root.mask._x = this._x;
        _root.mask._y = this._y;
        oldLoaded = 0;
}
onClipEvent(enterFrame)
{
        loaded = Math.ceil(_root.getBytesLoaded()/_root.getBytesTotal()*100);
        for(i = oldLoaded; i < loaded; i++)
        {
                addSlice(dO*i);
        }
        oldLoaded = loaded;
}

I've created a movieclip with a donut graphic in it.
Registration points are centered.
And set the AS to the movieclip.
There is a second frame in the timeline with a photo in it.
Nothing happens. Any suggestions?