Getting a value from a dropdown list mvc 4

I've come to the Council of Wise Wizards here at Stackoverflow because I need some magic (it seems) to fix a minor issue with my code.

I am attempting to receive data from my dropdown list, I've tried many spells... But I feel my powers are too weak. My attempts to retrieve data from the dropdown list results in my computer throwing up a little bit of sick. So I've come to you guys with hopes that your wiseness (is that even a word?) can help me to conjure up the correct code to defeat this issue.

Here's what my code looks like

The model (not very pretty for a model):

namespace HouseGaurd.Models
{
    public class DropList
    {
        public SelectList LettingList { get; set; }
    }

The controller fills up the list: (this is Httpget)

    public ActionResult lettingselect()
    {
         var model = _db.EstateAgents;
        DropList rlist = new DropList();
        List<SelectListItem> listItems = new List<SelectListItem>();
        foreach (var item in model)
        {

            listItems.Add(new SelectListItem()
            {
                Value = item.estateAgentID.ToString(),
                Text = item.CompanyName
            });
        }
        rlist.LettingList = new SelectList(listItems, "Value", "Text");
        return View(rlist);
    }

So this populates the list, this is the model that displays, so far everything works fine, the spell holds true:

<h2>Select a </h2>
@using (Html.BeginForm()) {

@Html.DropDownList("product", Model.LettingList)
  <p>
        <input type="submit" value="select" />
    </p>
}

It populates the dropdown list with the relevant information!

Now when I hit submit and we go back to the POST controller, this is when the computer starts to throw up:

[HttpPost]
    public ActionResult lettingselect(DropList viewModel)
    {           
        using (var context = new HouseDB())
        {
            var agent = viewModel.LettingList.DataTextField; //null reference
            var hID = context.HouseListers.Single(d=>d.user == User.Identity.Name);
            var let = new Letting();
            let.approved = false;
            let.user = User.Identity.Name;
            let.lettingAgent = agent;
            let.houseID = hID.houseID;
            let.noProblemsRaised = 0;

            context.Lettings.Add(let);
            context.SaveChanges();
        }
        return RedirectToAction("Complete");
    }

I'm trying to get the selected value back, I tried using:

 var agent = viewModel.LettingList.DataTextField; //null reference

But as stated in the code, this comes back with a null reference exception.

Wise Wizards, take a moment to share your wisdom with me on how to best approach this problem.

Many thanks. Apprentice wizard.

EDIT: this is the actual error:

Object reference not set to an instance of an object.

and this is the model in the view

@model HouseGaurd.Models.DropList

Could this be causing the problem?