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;
}

}