动作脚本-吐温问题

以下代码返回以下运行时错误。为什么?

代码:

package
{
import flash.display.Sprite;
import fl.transitions.Tween;
import fl.transitions.easing.Regular;
import fl.transitions.TweenEvent;

public class Test extends Sprite
    {
    private var myNumber:Number;
    private var myTween:Tween;

    public function Test()
        {
        myNumber = -12;
        myTween = new Tween(this, "myNumber", Regular.easeIn, myNumber, 0, 5, true);
        myTween.addEventListener(TweenEvent.MOTION_CHANGE, tweenChangeEventHandler);
        }

    private function tweenChangeEventHandler(evt:TweenEvent):void
        {
        trace(myNumber)
        }
    }
}

运行时错误:

ReferenceError: Error #1056: Cannot create property myNumber on Test.
    at fl.transitions::Tween/setPosition()
    at fl.transitions::Tween/set position()
    at fl.transitions::Tween()
    at Test()