MVC SelectList error

Question

I have a SelectList that I have passed into a form in my view.  When I try to post it I get this error:

There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key 'roleName'.

Code in Controller:

ViewBag.roleName = new SelectList(Roles.GetAllRoles().Where(rn =>

                rn != "Administrator" && rn != "Special"));

Code in View:

@Html.DropDownList("roleName", ViewBag.roleName as SelectList, "Select")