با سلام ممنون می شم دوستان کمکم کنید 
از لاین موجود در 
dropdownlist 
ارور می ده می گه
Object reference not set to an instance of an object
ممنون می شم کمکم کنید هر کار کردم نشد

TestController
 ----------------------
     [HttpGet]
        public ActionResult AddReseller()
        {
            var blOstan = new OstanRepository();
            var model = new MVCInternetShop.ViewModels.Test.AddRepositoryViewModel();
            model.Ostans = blOstan.Select().ToList();
            return View(model);
        }
        [HttpPost]
        public ActionResult AddReseller(Reseller reseller)
        {
            if (ModelState.IsValid)
            {
                var blReseller = new ResellerRepository();
                if (blReseller.Add(reseller))

                    ViewBag.Message = "نمایندگی ثبت شد";
                else
                    ViewBag.Message = "نمایندگی ثبت نشد";
            }
            else
                ViewBag.Message = "مقادر ورودی نا معتبر است";   
            return View();
        }

---------------------

view model

--------------------

using MVCInternetShop.Models.DomainModels;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace MVCInternetShop.ViewModels.Test
{
    public class AddRepositoryViewModel
    {
        public Reseller Reseller { get; set; }
        public IEnumerable<Ostan> Ostans { get; set; }
    }
}

----------------------------------

AddReseller view

--------------------------------------

@model MVCInternetShop.ViewModels.Test.AddRepositoryViewModel

@{
    ViewBag.Title = "AddReseller";
}

<h2>@ViewBag.Message</h2>

@using (Html.BeginForm("AddReseller","Test",FormMethod.Post))
{
    @Html.AntiForgeryToken()

    <div class="form-horizontal">
        <h4>Reseller</h4>
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        <div class="form-group">
            @Html.LabelFor(model => model.Reseller.Name, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.Reseller.Name, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.Reseller.Name, "", new { @class = "text-danger" })
            </div>
        </div>
        <div class="form-group">
            @Html.LabelFor(model => model.Reseller.Address, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.Reseller.Address, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.Reseller.Address, "", new { @class = "text-danger" })
            </div>
        </div>
        <div class="form-group">
            @Html.LabelFor(model => model.Reseller.OstanId, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownListFor(model => model.Reseller.OstanId, new SelectList(Model.Ostans, "Id", "Name"), "استان مورد نظر خود را انتخاب کنید", null)
                @Html.ValidationMessageFor(model => model.Reseller.OstanId, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Create" class="btn btn-default" />
            </div>
        </div>
    </div>
}