Error 500 when using Fiddler to POST to ASP.NET Web API

Question

I'm trying to learn how to create my own Web API with Visual Studio 2012. I currently have two database tables set up through the Models I have below.

The Countries table is nice and simple. I can successfully POST a new entry with Fiddler. However, when it comes to trying to POST to the Players table, I'm getting a 500 Error. Of course, I have no idea where I'm going wrong.

Player.cs

public class Player
{
    public Guid PlayerId { get; set; }
    [Required]
    public string Username { get; set; }
    [Required]
    public string Address { get; set; }
    public string Address2 { get; set; }
    [Required]
    public string Town { get; set; }
    [Required]
    public string State { get; set; }
    [Required]
    public string Postcode { get; set; }
    [Required]
    public int CountryId { get; set; }
    [Required]
    public string Email { get; set; }
    public string Telephone { get; set; }
    public string Mobile { get; set; }
    public DateTime? DateJoined { get; set; }

    // Navigation properties
    public Country Country { get; set; }
}

Countries.cs

    public class Country
{
    public int CountryId { get; set; }
    public string Name { get; set; }
}

I have the following Content Type set in the Fiddler Request Header:

Content-Type: application/json; charset=utf-8

The data I'm trying to POST to the Player table is this:

{"PlayerId":"hfjdfk","Username":"TickledPink","Address":"My Address","Address2":"","Town":"Llanerchymedd","State":"Anglesey", "Postcode":"LL71","Email":"myemail@me.net","Telephone":"1234567","Mobile":"456789","DateJoined":"","CountryId":"1"}

Answer 1

I know little to nothing about Web API, but is "hfjdfk" a valid GUID?