关于动态绑定的ArgumentNullException

Question

我正在尝试从数据库中获取最后100行并在datatable中显示它。但是我这样做了Argument.NullException。我该如何解决这个问题？

视图模型

public class WalkInnVM
{  
    ...
    ...
    public class WalkInnDataTable
    {
        public string SlNo { get; set; }
        public string Name { get; set; }
        public string Center { get; set; }
        public string Mobile { get; set; }
        public string CareGiverMobileNo { get; set; }
        public string Qualification { get; set; }
        public string CourseRecommended { get; set; }
        public int Id { get; set; }
    }     
}

控制器类

  WalkInnVM.WalkInnDataTable _dTableWalkInn = new WalkInnVM.WalkInnDataTable();
   if (_currentRole == (int)EnumClass.Role.SALESINDIVIDUAL)
            {
                _dTableWalkInn = _db.StudentWalkInns
                                 .AsEnumerable()
                                 .Where(x => x.ProspectHandledEmpID == Int32.Parse(Session["LoggedUserId"].ToString()))
                                 .OrderByDescending(x => x.Id)
                                 .Select(x => new WalkInnVM.WalkInnDataTable
                                 {
                                     SlNo = "",
                                     Name = x.CandidateName,
                                     Center = x.CenterCode.CentreCode,
                                     Mobile = x.MobileNo,
                                     CareGiverMobileNo = x.GuardianType == (int)EnumClass.CareGiver.Father ? "FATHER :" + x.GuardianContactNo :
                                                       x.GuardianType == (int)EnumClass.CareGiver.Guardian ? "GUARDIAN :" + x.GuardianContactNo :
                                                       x.GuardianType == (int)EnumClass.CareGiver.Mother ? "MOTHER :" + x.GuardianContactNo :
                                                                       "SPOUSE :" + x.GuardianContactNo,
                                     Qualification = x.QlfnMain.Name + "," + x.QlfnSub.Name,
                                     CourseRecommended = x.StudentWalkInnCourses
                                                      .Select(c => c.Course.Name)
                                                      .Aggregate((m, n) => m + "," + n),
                                     Id = x.Id
                                 })                                     
                                 .Take(100);                                                     
            }    

此处Take(100)显示ArgumentNullException，使用var变量而不是typos时问题得以解决。