The ViewData item that has the key 'MY KEY' is of type 'System.String' but must be of type 'IEnumerable<SelectListItem>'

I am trying to populate a dropdown list from a database mapped with Linq-2-SQL, using ASP.NET MVC 2, and keep getting this error.

I am so confused because I am declaring a variable of type IEnumerable<SelectListItem> on the second line, but the error makes me think this is not the case. I feel like this should be very simple, but I am struggling. Any help is appreciated.

Here are the interesting bits of my controller:

public ActionResult Create()
{
    var db = new DB();
    IEnumerable<SelectListItem> basetypes = db.Basetypes.Select(
        b => new SelectListItem { Value = b.basetype, Text = b.basetype });
    ViewData["basetype"] = basetypes;
    return View();
}

And here are the interesting bits of my view:

<div class="editor-label">
   <%: Html.LabelFor(model => model.basetype) %>
</div>
<div class="editor-field">
   <%: Html.DropDownList("basetype") %>
   <%: Html.ValidationMessageFor(model => model.basetype) %>
</div>

Here is the POST action when submitting the Form

// POST: /Meals/Create
[HttpPost]
public ActionResult Create(Meal meal)
{
    if (ModelState.IsValid)
    {
        try
        {
            // TODO: Add insert logic here
            var db = new DB();
            db.Meals.InsertOnSubmit(meal);
            db.SubmitChanges();
            return RedirectToAction("Index");
        }
        catch
        {
            return View(meal);
        }
    }
    else
    {
        return View(meal);
    }
}

Thanks.