override of Vector.prototype.toJSON results unexpected

worked this out from http://help.adobe.com/en_US/as3/dev/WS324d8efcab3b0d1e2408f9e3131fddffcfc-8000.html#WSab2db1cbc17f4fe03862fd0a132e04d0ed0-8000

the secret is to return an object and not to try and construct teh string yourself

package
{
          import flash.display.Sprite;
          import flash.events.Event;
          public class Main extends Sprite
          {
                    public function Main():void
                    {
                              if (stage)
                              {
                                        init();
                              }
                              else
                              {
                                        addEventListener(Event.ADDED_TO_STAGE, init);
                              }
                    }
                    public function init(e:Event = null):void
                    {
                              removeEventListener(Event.ADDED_TO_STAGE, init);
                              var d:MyVector3D = new MyVector3D();
                              trace(JSON.stringify(d));
                    }
          }
}

package  
{
          import flash.geom.Vector3D;
          public class MyVector3D extends Vector3D 
          {
                    public function MyVector3D() 
                    {
                    }
                    public function toJSON(s:String):*
                    {
                              return { x:x, y:y, z:z };
                    } 
          }
}