fail build ScriptableObject into bundle

I need Build ScriptableObject into bundle. But it is always null when www.assetBundle.mainAsset：sd

      [MenuItem("Assets/Export")]
     public static void Execute()
     {
         SysData sd = ScriptableObject.CreateInstance<SysData>();
         sd.content = "!!!!!!";
         string p = "Assets/SysData.asset"; 
         AssetDatabase.CreateAsset(sd, p);
         Object o = AssetDatabase.LoadAssetAtPath(p, typeof(SysData));
         BuildPipeline.BuildAssetBundle(o, null, Application.dataPath + "/ZZBundle/SysData.assetbundle", s_nBuildOptions,BuildTarget.StandaloneWindows);
          AssetDatabase.Refresh();
     }
 
 public class SysData:ScriptableObject
 {
     public string content;
 }

 IEnumerator LoadData()
     {
         WWW www = new WWW("file://" + Application.dataPath + "/ZZBundle/SysData.assetbundle");
        
         yield return www;
         SysData sd = www.assetBundle.mainAsset as SysData;
         Debug.Log(sd.content);
     }