asp.net-mvc-4 – MVC 4 DropDownListFor错误 – 没有具有密钥的

public class Auction
{
    public string Title { get; set; }
    public string category { get; set; }
}

和控制器：

[HttpGet]
public ActionResult UserForm()
{

    var categoryList = new SelectList(new[] { "auto","elec","games","Home" });
    ViewBag.categoryList = categoryList;
    return View();

}

在视图中我有以下几行：

<div class="editor-field">
    @Html.DropDownListFor(model =>
        model.category,(SelectList)ViewBag.categoryList)
    @Html.ValidationMessageFor(model => model.category)

</div>

我尝试保存表单时遇到的错误是：

There is no ViewData item of type ‘IEnumerable’ that
has the key ‘category’. 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.InvalidOperationException: There is no
ViewData item of type ‘IEnumerable’ that has the key
‘category’.

我不明白是什么问题,因为我做了(或试图做)本指南中完成的所有事情：
https://www.youtube.com/watch?v=7HM6kDBj0vE

该视频也可以在此链接中找到(第6章 – 自动绑定到请求中的数据)：
http://www.lynda.com/ASPNET-tutorials/ASPNET-MVC-4-Essential-Training/109762-2.html