i'm trying to assign a parent's variable from the parent's child

//Parent
public class Main extends Sprite
    {   
    public var selectedSquare:Sprite;

    public function Main()
        {
        //inits and adds new Square child class to display list
        }
...

-------

//Child
public function dragSquare(evt:MouseEvent):void
    {
    Sprite(parent).selectedSquare = this; //evil doesn't work!
    parent.addChild(this);
    this.startDrag();
    }

i'm receiving this error, but i'm casting parent from displayObjectContainer to a Sprite so i have no idea why it's not working.

1119: Access of possibly undefined property selectedSquare through a reference with static type flash.display:Sprite.