HElp! why I assign eventListener isn't working

1

I draw a circleshape as Sprite and add it to stage … Then addEventListener. not working.??? WHY WHY?

package {
import flash.display.Sprite;
import flash.display.MovieClip;
import flash.display.Shape;
import flash.events.MouseEvent;

public class CircleShape extends Sprite
{
public function CircleShape()
{

var myCircle:Sprite = new Sprite();
myCircle.graphics.beginFill(0xFF, 1);
myCircle.graphics.drawCircle(40, 40, 20);
addChild(myCircle);

myCircle.addEventListener(MouseEvent.CLICK, clickButton)
myCircle.buttonMode = true;

function clickButton ($evt:MouseEvent):Void
{
myCircle.alpha = .5;
}

}

}
}

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Thanks for any help.

NB