Error Message : Event Handler

programming flash
1

Hi.
I have a code designed to move a ball on screen according to mouse positions, but 6 errors are compiled and all say: 1119: Access of possibly undefined property MOUSE_DOWN through a reference with static type Class.
for each addEventListener

Code:

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

public class MouseEvent extends Sprite {
	private var ball:Ball;
	
	public function MouseEvent() {
		init();
	}
	
	private function init():void {
		var ball:Ball = new Ball();
		ball.x = 100;
		ball.y = 100;
		addChild(ball);
		
		ball.addEventListener(MouseEvent.MOUSE_DOWN, onMouseDownBall);
		ball.addEventListener(MouseEvent.MOUSE_UP, onMouseUpBall);
		ball.addEventListener(MouseEvent.MOUSE_MOVE, onMouseMoveBall);
		stage.addEventListener(MouseEvent.MOUSE_DOWN, onMouseDownStage);
		stage.addEventListener(MouseEvent.MOUSE_UP, onMouseUpStage);
		stage.addEventListener(MouseEvent.MOUSE_MOVE, onMouseMoveStage);
		
	}
	
	private function onMouseDownBall(event:MouseEvent):void {
		trace("mouse down – ball");
	}
	
	private function onMouseUpBall(event:MouseEvent):void {
		trace("mouse down – ball");
	}
	
	private function onMouseMoveBall(event:MouseEvent):void {
		trace("mouse move – ball");
	}
	
	private function onMouseDownStage(event:MouseEvent):void {
		trace("mouse down – stage");
	}
	
	private function onMouseUpStage(event:MouseEvent):void {
		trace("mouse up – stage");
	}
	
	private function onMouseMoveStage(event:MouseEvent):void {
		trace("mouse move – stage");
	}
}

}

Thanks.