MVC 4 - how to fix error Cannot implicitly convert type 'System.Linq.IOrderedQueryable' to 'System.Data.Entity.DbSet'.

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Error message

Cannot implicitly convert type 'System.Linq.IOrderedQueryable<MVCTestRizne.Models.Dpt>' to 'System.Data.Entity.DbSet<MVCTestRizne.Models.Dpt>'.
An explicit conversion exists (are you missing a cast?)    C:\LocalDoc\VS2012Projects\Test\MVCTestRizne\MVCTestRizne\Controllers\DptController.cs


Replace

        public ActionResult Index(string SortBy)
        {
            ViewBag.NameSortParam = String.IsNullOrEmpty(SortBy) ? "Name Desc" : "";
         //   return View(db.Dpts.ToList());
            var departments = db.Dpts;

            switch (SortBy)
            {
                case "Name Desc":
                    departments = departments.OrderByDescending(s => s.DepartmentName);
                    break;
                default:
                    departments = departments.OrderBy(d => d.DepartmentName);
                    break;
            }
            return View(departments.ToList());
        }

with

    public ActionResult Index(string SortBy)
        {
            ViewBag.NameSortParam = String.IsNullOrEmpty(SortBy) ? "Name Desc" : "";
         //   return View(db.Dpts.ToList());
            var departments = db.Dpts.Include(u => u.Users);

            switch (SortBy)
            {
                case "Name Desc":
                    departments = departments.OrderByDescending(s => s.DepartmentName);
                    break;
                default:
                    departments = departments.OrderBy(d => d.DepartmentName);
                    break;
            }
            return View(departments.ToList());
        }