Выпадающий список для nullable int

Сайт на asp.net core 2.0, нужно сделать CRUD-контроллер для такой сущности:

public class Season
{
    public int Id { get; set; }
    [Required]
    [MaxLength(50)]
    public string Title { get; set; }
    public DateTime StartDate { get; set; }
    public DateTime EndDate { get; set; }
    public int? ParentSeasonId { get; set; }
    public Season ParentSeason { get; set; }
}

Стандартный код контроллера, который предлагает студия почему-то запрещает ввод NULL значения в поле parent season.

Сам контроллер:

// GET: Season/Create
public IActionResult Create()
{
    this.ViewData["ParentSeasonId"] = new SelectList(this._context.Seasons, "Id", "Title");
    return this.View();
}
// POST: Season/Create
// To protect from overposting attacks, please enable the specific properties you want to bind to, for 
// more details see http://go.microsoft.com/fwlink/?LinkId=317598.
[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create([Bind("Id,GlobalNum,Title,StartDate,EndDate,ParentSeasonId")] Season season)
{
    if (this.ModelState.IsValid)
    {
        this._context.Add(season);
        await this._context.SaveChangesAsync();
        return this.RedirectToAction(nameof(this.Index));
    }
    this.ViewData["ParentSeasonId"] = new SelectList(this._context.Seasons, "Id", "Title", season.ParentSeasonId);
    return this.View(season);
}
// GET: Season/Edit/5
public async Task<IActionResult> Edit(int? id)
{
    if (id == null)
    {
        return this.NotFound();
    }
    var season = await this._context.Seasons.SingleOrDefaultAsync(m => m.Id == id);
    if (season == null)
    {
        return this.NotFound();
    }
    this.ViewData["ParentSeasonId"] = new SelectList(this._context.Seasons, "Id", "Title", season.ParentSeasonId);
    return this.View(season);
}
// POST: Season/Edit/5
// To protect from overposting attacks, please enable the specific properties you want to bind to, for 
// more details see http://go.microsoft.com/fwlink/?LinkId=317598.
[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Edit(int id, [Bind("Id,GlobalNum,Title,StartDate,EndDate,ParentSeasonId")] Season season)
{
    if (id != season.Id)
    {
        return this.NotFound();
    }
    if (this.ModelState.IsValid)
    {
        try
        {
            this._context.Update(season);
            await this._context.SaveChangesAsync();
        }
        catch (DbUpdateConcurrencyException)
        {
            if (!this.SeasonExists(season.Id))
            {
                return this.NotFound();
            }
            else
            {
                throw;
            }
        }
        return this.RedirectToAction(nameof(this.Index));
    }
    this.ViewData["ParentSeasonId"] = new SelectList(this._context.Seasons, "Id", "Title", season.ParentSeasonId);
    return this.View(season);
}

Представления:

<form asp-action="Create">
    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
    <div class="form-group">
        <label asp-for="Title" class="control-label"></label>
        <input asp-for="Title" class="form-control" />
        <span asp-validation-for="Title" class="text-danger"></span>
    </div>
    <div class="form-group">
        <label asp-for="StartDate" class="control-label"></label>
        <input asp-for="StartDate" class="form-control" />
        <span asp-validation-for="StartDate" class="text-danger"></span>
    </div>
    <div class="form-group">
        <label asp-for="EndDate" class="control-label"></label>
        <input asp-for="EndDate" class="form-control" />
        <span asp-validation-for="EndDate" class="text-danger"></span>
    </div>
    <div class="form-group">
        <label asp-for="ParentSeasonId" class="control-label"></label>
        <select asp-for="ParentSeasonId" class ="form-control" asp-items="ViewBag.ParentSeasonId"></select>
    </div>
    <div class="form-group">
        <input type="submit" value="Create" class="btn btn-default" />
    </div>
</form>