protected setter error

/**
 * Copyright rch850 ( http://wonderfl.net/user/rch850 )
 * MIT License ( http://www.opensource.org/licenses/mit-license.php )
 * Downloaded from: http://wonderfl.net/c/8gOv
 */

package {
    import flash.display.Sprite;
    public class FlashTest extends Sprite {
        public function FlashTest() {
            this.radius = 100;
            this.graphics.beginFill(0xFF0000);
            this.graphics.drawCircle(100, 100, _radius);
        }
    
        private var _radius:Number = 10;
        public function get radius():Number { return _radius; }
        protected function set radius(n:Number):void { _radius = n; }
    }
    
}