Droptarget 1009 error

1

I am teaching with an basic AS3 programming class and we are looking at drag/drop. I cannot get the following code to work without a 1009 error when not dropping the ball into the inventory box.

What we are trying to accomplish is creating an inventory for an adventure game.

function ball_drag(event:MouseEvent):void {
           ball.startDrag();
           }
ball.addEventListener(MouseEvent.MOUSE_DOWN,ball_drag);

function ball_drop(event:MouseEvent):void {
           stopDrag();
           if (ball.dropTarget.parent == inventory_box) {
                             trace("it worked!");
           }
    }

ball.addEventListener(MouseEvent.MOUSE_UP,ball_drop);

Thanks in advance, file is attached.