TypeError: Error #1009: Cannot access a property or method of a null object reference

THis is weird!! I dont know what happen!

I try to load an external swf using Loader.

this is my code

package {
	import flash.display.Loader;
	import flash.display.MovieClip;
	import flash.net.URLRequest;
	 * ...
	 * @author 4 loop
	public class Main extends MovieClip 
		public function Main():void 
			var myLoader:Loader = new Loader();
			myLoader.load(new URLRequest("DigitalMap.swf"));

and I got an error

but when I change myLoader.load(new URLRequest(“DigitalMap.swf”)); into myLoader.load(new URLRequest(“a.swf”)); its ok…

They are all in the same location!

see this… whats going on guys?? I cant figure it out!

Help me guys! I am making preloader that loads another swf file…