asp.net mvc4 在EF新增的时候报对一个实体或多个实体验证失败

//entity为空 是数据库上下文会验证实体验证
                var entity = db.UserInfo.Where(u => u.Mobile == mobile).FirstOrDefault();
 
                if (entity != null)
                {
 
                    MobileLogModel.AddMobileLog(dvid, dvType, "用户注册", "手机已注册", "UserInfo-Register");
 
                    return Json(result, JsonRequestBehavior.AllowGet);
                }
 
  public static void AddMobileLog( string title, string msg)
        {
            using (Dao.GiftDBEntities db = new Dao.GiftDBEntities())
            {
               //让db 不对实体验证
                db.Configuration.ValidateOnSaveEnabled = false;
 
                Dao.MobileLog log = new Dao.MobileLog();
 
                log.MobileLogID = Utility.GetGUID();
 
                log.Createtime = DateTime.Now;
                log.Title = title;
                log.Content = msg;
 
                db.MobileLog.Add(log);
 
                db.SaveChanges();
 
    //再让db对实体验证
                db.Configuration.ValidateOnSaveEnabled = true;
            }
 
        }