从Html.BeginForm传递时，参数returnUrl为null

Question

基于我在stackoverflow中看到的内容，我有一个Html.BeginForm：

@using (Html.BeginForm("Index", "Account", new {returnUrl = ViewBag.ReturnUrl}, FormMethod.Post))
{
    @Html.AntiForgeryToken()

    @Html.ValidationSummary(true, "", new {@class = "text-danger"})

    @Html.LabelFor(model => model.Username, htmlAttributes: new { @class = "control-label col-md-2" })
    @Html.EditorFor(model => model.Username)
    @Html.ValidationMessageFor(model => model.Username, "", new {@class = "text-danger"})

    <br/>

    @Html.LabelFor(model => model.Password, htmlAttributes: new { @class = "control-label col-md-2" })
    @Html.PasswordFor(model => model.Password)
    @Html.ValidationMessageFor(model => model.Password, "", new {@class = "text-danger"})

    <br/>

    <input type="submit" value="Log in" />
}

在调试模式下，我在Controller中的returnUrl上得到一个空值，但模型已填满。

这是我的Web.config

<system.web>
<globalization culture="en-GB" uiCulture="en-GB"/>
<authentication mode="Forms">
  <forms loginUrl="~/Account/Index" defaultUrl="~/Home/Default" timeout="20" slidingExpiration="true" />
</authentication>
<roleManager enabled="true" defaultProvider="MyProvider">
  <providers>
    <clear/>
    <add name="MyProvider" type="EventCalendar.MyRoleProvider.SiteRole"/>
  </providers>
</roleManager>
<compilation debug="true" targetFramework="4.5"/>
<httpRuntime targetFramework="4.5"/>
</system.web> 

我做错了什么？感谢。