Animation af mask

Hvorfor kan jeg ikke få dette script til at virke, når jeg sætter mc'et i et masked layer??

onClipEvent (mouseDown) {
    targetx = _parent._xmouse;
    if(targetx<0){
        targetx=0
    }else if(targetx>180){
        targetx=180
    }
    targety = _parent._ymouse;
    if(targety<0){
        targety=0
    }else if(targety>180){
        targety=180
    }

    active = true;
}
onClipEvent(enterFrame){

    if(active){
       
        xdif=targetx-_x
        xspeed+=xdif/2
        xspeed*=0.2
        _x+=xspeed

        ydif=targety-_y
        yspeed+=ydif/2
        yspeed*=0.2
        _y+=yspeed
   
        if(Math.abs(xspeed)<0.02 && Math.abs(yspeed)<0.02){
            delete active
            delete xspeed
            delete yspeed
            delete xdif
            delete ydif
        }
    }
}

Venlig hilsen
guster