ASP.NET MVC: System.ArgumentNullException: 'Value cannot be null. Parameter name: items' [duplicate]

I'm currently working on this assignment. Everything is working fine except when I put a drop-down list from another table. I need to insert a row in my database but I'm running across an error that says

System.ArgumentNullException: 'Value cannot be null.
Parameter name: items'

I've looked for the solutions I found online but for some reason it doesn't apply to my problem.

Controller:

[HttpGet]
    public ActionResult Add()
    {
        try
        {
            ViewBag.Departments = db.Departments.ToList();
            return View();
        }
        catch (Exception genericException) 
        {
            ViewBag.ExceptionMessage = genericException.Message;
        }
        return View("~/Views/Errors/Details.cshtml");
    }

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Add(Volunteer volunteer)
    {
        try
        {
            if (ModelState.IsValid)
            {
                db.Volunteers.Add(volunteer);
                db.SaveChanges();
                return RedirectToAction("Index");
            }
            ViewBag.Department = db.Departments.ToList();
            return View(volunteer);
        }
        catch (DbUpdateException DbException)
        {
            ViewBag.DbExceptionMessage = DbException.Message;
        }
        catch (Exception genericException)
        {
            ViewBag.ExceptionMessage = genericException;
        }
        return View("~/Views/Errors/Details.cshtml");
    }

View:

@model Features.Models.Volunteer
@{
ViewBag.Title = "Add";
Layout = "~/Views/Shared/_Layout.cshtml";
}

<h2>Add a volunteer</h2>

@using (Html.BeginForm("Add", "Volunteer"))
{
@Html.AntiForgeryToken()

<div>
    <div>
        @Html.LabelFor(v => v.Department)

        @Html.DropDownListFor(v => v.Department, new SelectList(ViewBag.Department, "DepartmentName", "DepartmentName"), "-")
        @Html.ValidationMessageFor(v => v.Department)
    </div>
</div>
<input type="submit" value="Create" />

Can someone help me please? Thank you.