asp.net – 值不能为null.参数名称：items(DrodownList)

在我看来(Register.cshtml)：`

<div class="control-group">
    @Html.LabelFor(m => m.DistrictID,new { @class= "control-label"})
    <div class="controls">
        @Html.DropDownListFor(model => model.DistrictID,new SelectList(ViewBag.Districts,"district_id","district_name",Model.DistrictID))
    </div>
</div>

在我的Controller(AccountController)中：

[AllowAnonymous]
public ActionResult Register()
{
    var districts = repository.GetDistricts();
    ViewBag.Districts = districts;

    return View(new RegisterModel());
}

//
// POST: /Account/Register

[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
public ActionResult Register(RegisterModel model)
{

    if (ModelState.IsValid)
    {
        // Attempt to register the User
        try
        {
            MembershipService.CreateUser(model.Name,model.FamilyName,model.BirthDate,model.Sex,model.Nationality,model.Email,model.UserName,model.Password,model.Street,model.StreetNr);

            FormsAuthentication.SetAuthCookie(model.UserName,false);
            return RedirectToAction("Index","Home");
        }
        catch (ArgumentException ae)
        {
            ModelState.AddModelError("",ae.Message);
        }
    }

    // If we got this far,something failed,redisplay form
    return View(model);
}

我从我的存储库中获取区域,如下所示：

public IQueryable<district> GetDistricts()
 {
     return from district in entities.districts
            orderby district.district_name
            select district;
 }

我的RegisterModel：

public class RegisterModel
{
    [Required]
    [Display(Name = "Given name")]
    public string Name { get; set; }

    [Required]
    [Display(Name = "Family name")]
    public string FamilyName { get; set; }

    [Required]
    [Display(Name = "Birthdate")]
    public DateTime BirthDate { get; set; }

    [Required]
    [Display(Name = "Sex")]
    public string Sex { get; set; }

    [Required]
    [Display(Name = "Nationality")]
    public string Nationality { get; set; }

    [Required]
    [Display(Name = "Email")]
    public string Email { get; set; }

    [Required]
    [Display(Name = "User name")]
    public string UserName { get; set; }

    [Required]
    [StringLength(100,ErrorMessage = "The {0} must be at least {2} characters long.",MinimumLength = 6)]
    [DataType(DataType.Password)]
    [Display(Name = "Password")]
    public string Password { get; set; }

    [DataType(DataType.Password)]
    [Display(Name = "Confirm password")]
    [Compare("Password",ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }

    [Required]
    [Display(Name = "Street")]
    public string Street { get; set; }

    [Required]
    [Display(Name = "Street Number")]
    public int StreetNr { get; set; }

    [Required]
    [Display(Name = "District")]
    public IEnumerable<SelectListItem> Districts { get; set; }

    public int DistrictID { get; set; }
}

下拉列表中有区域,但是当我点击“注册”时,我收到此错误：

Value cannot be null.
Parameter name: items

Description: An unhandled exception occurred during the execution of
the current web request. Please review the stack trace for more
information about the error and where it originated in the code.

Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: items

当我调试方法时,我看到ModelState无效.
关键11是DistrictID,包括districtid但Key 12是Districts并给出错误：“区域字段是必需的”……

我究竟做错了什么？