scriptableobject assetbundle error in Unity5

I Created a scriptableobject asset："test.asset"

     public class MYtest : ScriptableObject
     {
         public string Name;
         private int aaa;
         public int AAA
         {
             get { return aaa; }
             set { aaa = value; }
         }
     }
 
 // function
    MYtest mt = ScriptableObject.CreateInstance<MYtest>();
    mt.AAA = 199999;
    mt.Name = "Hello";
    AssetDatabase.CreateAsset(mt,"mytest.asset");
    AssetDatabase.SaveAssets();
 
 /****************/
 //Loaded AssetDatabase.LoadAssetAtPath
 var mt = AssetDatabase.LoadAssetAtPath("mytest.asset", typeof(MYtest));
 //mt.AAA ->199999
 //mt.Name ->Hello
 
 /****************/
 //Loaded WWW
 string url = @"file:///E:\Test\mytest.unity3d";
 WWW _www = new WWW(url);
 yield return _www;
 MYtest mt = _www.assetBundle.LoadAsset<MYtest>("mytest");
 //mt.AAA ->0              ERROR!!!
 //mt.Name ->Hello   Right!

When I loaded it use "AssetDatabase.LoadAssetAtPath();", that is right. When I build a assetbundle and loaded is use "WWW",it's field is right,but it's property is error.