movieclip setMask problem

Can anybody explain me when I use a variable string as movieclipname to mask It won't work?

Doesn't work:

var eraserId = "test"
this.attachMovie("eraser",eraserId,this.getNextHighestDepth(), {_x:_xmouse, _y:_ymouse});
this.setMask(eraserId);

Does work:

this.attachMovie("eraser","test",this.getNextHighestDepth(), {_x:_xmouse, _y:_ymouse});
this.setMask(test);

Thanks for your help,
Rick