Bad request (400) when using PutAsJsonAsync if Model has navigation properties

I am creating a simple API and I have a method that handles the update of a game:

    [HttpPut("{Id}")]
    public bool UpdateGame(Guid Id, [FromBody] Game game)
    {
        Game? oldGame = _unitOfWork.Games.GetById(Id);
        if (oldGame != null)
        {
            oldGame.SeasonId = game.SeasonId;
            oldGame.VenueId = game.VenueId;
            oldGame.GameTypeId = game.GameTypeId;
            oldGame.GameTitle = game.GameTitle;
            oldGame.GameDateTime = game.GameDateTime;
            oldGame.PublishResults = game.PublishResults;
            oldGame.GameDetails = game.GameDetails;
            oldGame.Buyin = game.Buyin;
            oldGame.Fee = game.Fee;
            _unitOfWork.Save();
            return true;
        }
        else
        {
            return false;
        }
    }

I have tests against the repo and the controller and the update works fine.

I'm now implementing some CRUD pages using Blazor Server:

    async Task OnSubmit()
    {
        if (Game != null)
        {
            try
            {
                var response = await _apiClient.httpClient.PutAsJsonAsync<Game>($"/api/Games/{Id}", Game);
                bool updated = await response.Content.ReadFromJsonAsync<bool>();

                if (updated)
                {
                    _navManager.NavigateTo("/settings/games");
                }
            }
            catch (Exception ex)
            {
                AlertIsVisible = true;
                Message = ex.Message;
                MessageType = AlertMessageType.Danger;
            }
        }
    }

This is failing because of a bad request 400, but this is all the output is telling me. The put method in the controller doesn't hit as my breakpoint doesn't trigger.

Simpler models in my app post just fine using similar code.

The difference here I believe is this Game entity:

public class Game
{
    public Guid Id { get; set; }
    public Guid? SeasonId { get; set; }
    public Guid? GameTypeId { get; set; }
    public Guid? VenueId { get; set; }
    public string? GameTitle { get; set; }
    public DateTime GameDateTime { get; set; }
    public bool PublishResults { get; set; }
    public string? GameDetails { get; set; }
    public double Buyin { get; set; }
    public double Fee { get; set; }

    public virtual Season Season { get; set; } = default!;
    public virtual GameType GameType { get; set; } = default!;
    public virtual Venue Venue { get; set; } = default!;
    public virtual ICollection<Result> Results { get; set; } = new HashSet<Result>();
}

I think there is a serialisation issue with the navigation properties (Season, GameType and Venue - not the Results collection). If I remove the navigation properties the update succeeds. I'm at a loss at this point on how to handle the put request so that it works with the navigation properties on my model.

You declare Season, GameType and Venue as not Nullable [no ?], and then forcibly set to default!, which for any object is null.

You normally use default! when you know an object will be set to a value before any attempt is made to use it, which is not the case here.