ASP.Net MVC Error: There is no ViewData item of type IEnumerable that has the key username

I have a dropdownlist connected to a viewbag list and the correct list is displayed in the ddl. When I click my btn for a postback I get

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

I tried without a 'IEnumerable or just a SelectList() a few different ways and I haven't been able to get my method to work GetRolesForUser.

Any input would be appreciated. Here is what I have. 

                @using (Html.BeginForm("GetSelectedUserRole", "SuperAdmin"))
                {
                    @Html.AntiForgeryToken()
                    <div class="panel-body">
                        <div class="input-group">
                            <span class="input-group-addon">User Name</span>
                            <span class="input-group-addon">
                                <i class="glyphicon glyphicon-user"></i>
                            </span>
                            @Html.DropDownList("username",(IEnumerable<SelectListItem>)ViewBag.SelectUsers, "Choose ...",new { @class = "form-control" })
                        </div>
                    </div>
                    <div class="panel-footer">
                        <input type="submit" value="Get Role" class="btn btn-primary" />
                    </div>
                }
                @if (ViewBag.RolesForUser != null)
                {
                    <div class="alert alert-info text-center">
                        <strong>Role for User</strong>
                        <ol style="list-style: decimal inside;">
                            @foreach (var role in ViewBag.Roles4User)
                            {
                                <li>@role</li>
                            }
                        </ol>
                    </div>
                } 

        public void UsersSelectList()
        {
            var usersList = (from users in context.Users
                             select new SelectListItem()
                             {
                                 Value = users.UserName.ToString(),
                                 Text = users.UserName
                             }).ToList();
            ViewBag.SelectUsers = usersList;
        }
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult GetSelectedUserRole(string username)
        {
            AssignRolesModel rolesModel = new AssignRolesModel
            {
                lstAdmins = AdminList(),
                lstUsers = UserList()
            };

            if (!string.IsNullOrWhiteSpace(username))
            {
                var store = new UserStore<ApplicationUser>(context);
                var manager = new UserManager<ApplicationUser>(store);
                var user = manager.FindByName(username);
                var role = manager.GetRoles(user.Id).FirstOrDefault();
                if(role != null)
                {
                    ViewBag.Roles4User = role;
                    TempData["Success"] = "Roles Retrieved Successfully";
                    return View("AssignAdmin", rolesModel);
                }
                TempData["Error"] = "Role Retrieval Unsuccessful";
                return View("AssignAdmin", rolesModel);
            }
            else
            {
                TempData["Error"] = "Role Retrieval Unsuccessful";
                return View("AssignAdmin", rolesModel);
            }
        }