Newbie class problem

1

Hi there, i am trying to make a rollOver effect using class


package
{
    import flash.display.*;
    import flash.events.*;
    import flash.text.*;
    
    public class MainBtn extends MovieClip
    {
        
        public function MainBtn()
        {
            this.buttonMode = true;
            this.tbtnName.text = this.name;
            this.tbtnName.autoSize = TextFieldAutoSize.LEFT;
            //
            //this.MainBtn_bg.width = this.tbtnName.width+8;
            //
            this.addEventListener(MouseEvent.ROLL_OVER, btnRollOver);
            this.addEventListener(MouseEvent.ROLL_OUT, btnRollOut);
        }
        
        function btnRollOver(event:MouseEvent):void
        {
            trace("rollOver");
            event.target.gotoAndPlay(2);
        }
        
        function btnRollOut(event:MouseEvent):void
        {
            trace("rollOut");
            event.target.gotoAndPlay(11);
        }
        
    }
    
}

when i comment this part

this.MainBtn_bg.width = this.tbtnName.width+8;

then it works fine. Else it resizes the button but doesnt not have the rollOver effect. Please help.