DropDownList使用viewmodel MVC4返回null

Question

视图模型：

namespace CFMembers.ViewModels
{
    public class IndexData
    {

        public LicenseHolder LicenseHolders { get; set; }
        public Installation Installations { get; set; }

    }
}

控制器：

public ActionResult Create()
{

        LicenseHolder licenseHolder = new LicenseHolder();



        IndexData ViewData = new IndexData
        {
            LicenseHolders = licenseHolder,

        };

        PopulatePartyTypeDropDownList();
        PopulateLicenseHolderTypeDropDownList();
        return View(ViewData);

    }

    //
    // POST: /Main/Create

    [ActionName("Create"), AcceptVerbs(HttpVerbs.Post)]
    public ActionResult CreateSubmit(IndexData viewModel)
    {


        if (ModelState.IsValid)
        {

            db.LicenseHolder.Add(viewModel.LicenseHolders);
            db.Installation.Add(viewModel.Installations);

            db.SaveChanges();
            return RedirectToAction("Index");
        }


        return View();
    }

Create.cshtml

<div class="editor-label">
    @Html.LabelFor(model => model.LicenseHolders.LicenseHolderTypeID, "License Type")
 </div>
 <div class="editor-field">
    @Html.DropDownList("LicenseHolderTypeID", String.Empty)
    @Html.ValidationMessageFor(model => model.LicenseHolders.LicenseHolderTypeID)               
</div>

@ Html.DropDownList在另一个视图上运行得很好。但是，这不适用于具有多个表的viewmodel。当它被发回时，LicenseHolderTypeID为0，无论我做什么。我也试过dropdownlistfor并且仍然得到0.任何助手都将不胜感激。我认为这与视图模型的传递方式有关，但我被卡住了。