Mask not working

1

I am making a mask following the tutorial on this website, but there must be something I’m not getting. Basically, the scrollmask mc is supposed to be masking the flashscroll mc but it is still visible and not acting as a mask.

this.createEmptyMovieClip("scrollmask", 6);
scrollmask.lineStyle(1, 0x666666);
scrollmask.beginFill(0x666666, 100);
scrollmask.moveTo(460, 0);
scrollmask.lineTo(640, 0);
scrollmask.lineTo(640, 300);
scrollmask.lineTo(460, 300);
scrollmask.lineTo(460, 0);
scrollmask.endFill();
attachMovie("flashscroll", "flashscroll", 5, {_x:480, _y:20});
flashscroll.setMask(scrollmask)

Thanks for any help.