AS3 sprite/interface issue

package
{
    import flash.display.Bitmap;
    import flash.display.Sprite;
    import flash.events.Event;

    import interfaces.ISprite;

    [SWF(width="600", height="400", backgroundColor="#cccccc", frameRate="60")]

    public class Main extends Sprite
    {
        private var _flakeList:Array;

        public function Main()
        {
            createWinter();
            createFlakes(5);
        }

        private function createWinter():void
        {
            var bg:Bitmap = new Bitmap(new Winter(0, 0) ); //instantiate bitmap
            this.addChild(bg); //add bitmap to screen
        }

        private function createFlakes(amt:int):void
        {
            _flakeList=[]; //instantiate array

            //loop to create flakes using var passed from above to determine how many
            for (var i:int=0; i<amt; i++)
            {
                var flake:Snowflake2 = new Snowflake2();
                flake.x = 50 + i*70; //position flakes across x axis
                flake.y = 50;
                flake.scaleX = flake.scaleY = .25; //scale flakes to 25%
                this.addChild(flake); //add flakes to screen
                _flakeList.push(flake);
            }
            this.addEventListener(Event.ENTER_FRAME, update);
        }

        private function update(e:Event):void
        {
            //trace("work");
            //loop thru array of flakes, run update for each one
            for each(var flake:ISprite in _flakeList)
            {
                flake.update();
            }
        }
    }
}