Even when I check for a value in my nullable parameter, the browser still gives me a 500 error

Hi, I am getting this browser console error if the mainGameId parameter is null:

POST https://example.com/api/devConnection/66544/jwt/null 500

But in my controller, I am checking it for null in here:

[HttpPost("{id}/Jwt/{mainGameId}")]
[ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
public JsonResult Jwt(Int32 id, Int32? mainGameId)
{
    var game = GameCollection.Get(id);
    var gameTitle = game.Title;
    string token;
    if(mainGameId.HasValue)
    {
         token = CommUtils.GenerateValidToken(mainGameId.ToString(), gameTitle);
    } 
    else
    {
         token = CommUtils.GenerateValidToken(id.ToString(), gameTitle);
    }

    return Json(new { token });
}

But even with that check, I still get the error above.

Is there any other type of check I need to do?

Thanks!