ActionScript 3.0 :: Triggering TweenLite - Mouseout Listener Error

I've got the following problem: I have a button with two listeners: a mouseover and a mouseout. When I mouseover I trigger a TweenLite x tween that tweens the position of a textfield inside the button. It tweens back on the mouseout.

This is the code:
btn.addEventListener(MouseEvent.MOUSE_OVER, onMouseOver);
btn.addEventListener(MouseEvent.MOUSE_OUT, onMouseOut);
private function onMouseOver(e:Event):void{
TweenLite.to(e.target.lbl_txt, .2, {x:12});
}private function onMouseOut(e:Event):void{
TweenLite.to(e.target.lbl_txt, .2, {x:8});
}

When I am already hovering the button before the listeners are set and then the listeners are set I get this error:
ReferenceError: Error #1069: Property lbl_txt not found on flash.text.TextField and there is no default value.
at packagename::Main/onMouseOut()