Warning 1090?

I’m having the following warning, I’m not really sure if it has any bad effect on my animations or if it matters but it would be really nice if I could fix it somehow:
[COLOR=Red]Warning: 1090: Migration issue: The onEnterFrame is not triggered automatically by Flash Player at run time in ActionScript 3.0. You must first register this handler for the event using addEventListener ( ‘enterFrame’, callback_handler).[/COLOR][/COLOR]

Could someone explain why am I getting that warning ?

If this helps, I’m just playing around in AS 3.0, here’s the code ( a class ):

package {
    import flash.display.Sprite;
    import flash.events.Event;
    public class FirstAnimation extends Sprite {
        private var ball:Sprite;
        public function FirstAnimation() {
        private function init():void {
            ball = new Sprite;
            ball.graphics.drawCircle(0, 0, 40);
            ball.x = 20;
            ball.y = stage.stageHeight / 2;
            ball.addEventListener(Event.ENTER_FRAME, onEnterFrame);
        private function onEnterFrame(event:Event):void {