ASP .NET MVC - Creating a partial view only works for 'Edit' actionresult and not 'Create'

Question

I have created a partial view for use by by the 'Edit' view of my model. I can successfully edit records but when using the partial view for my 'Create' view I get a null reference exception.

This is my partial view:

@model MvcA.Models.Reason       

        @Html.LabelFor(model => model.reason)
        @Html.EditorFor(model => model.reason)

        @Html.LabelFor(model => model.Contract)
        @Html.DropDownList("ContractId",
        new SelectList(ViewBag.Contract as System.Collections.IEnumerable,
       "ContractId","Name",Model.ContractID));

And POST ActionResult:

[HttpPost]
public ActionResult Create(Reason reason)
{
   if (ModelState.IsValid)
            {
                db.Reason.Add(reason);
                db.SaveChanges();
                return RedirectToAction("Index");
            }
            //invalid ...

The GET Create:

public ActionResult Create()
    {
        ViewBag.Contract = db.Contract.OrderBy(g => g.Name).ToList();
        var reason = new Reason();
        return View(reason);
    } 

Upon entering/selecting valid values the form submit will result in Visual Studio exiting out to the 'DropDownList' found in the partial view with a 'NullReferenceException was unhandled'.

How do I determine what is causing the null error? (I'm new to MVC)

UPDATE: The error appears to be related to the [HttpPost] Create method in my controller. I was naming the input class using the same name as one of the fields in the model...this appears to have broken the program with the null reference exception.