Serializing DbGeography Type to MVC

I have a problem serializing a DbGeography type. My controller action always gives an error instead of an object.

For example, here's my class:

public class Centre
{
  public int CentreId { get; set; }

  [Required]
  public string Name { get; set; }

  [Required]
  [JsonConverter(typeof(DbGeographyConverter))]
  public DbGeography Location { get; set; }
}

      

        
        
        
      

    

The My DbGeographyConverter class is defined as many examples:

public class DbGeographyConverter : JsonConverter
{
  private const string LATITUDE_KEY = "latitude";
  private const string LONGITUDE_KEY = "longitude";

  public override bool CanConvert(Type objectType)
  {
    return objectType.Equals(typeof(DbGeography));
  }

  public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
  {
    if (reader.TokenType == JsonToken.Null)
      return default(DbGeography);

    var jObject = JObject.Load(reader);

    if (!jObject.HasValues || (jObject.Property(LATITUDE_KEY) == null || jObject.Property(LONGITUDE_KEY) == null))
      return default(DbGeography);

    string wkt = string.Format(CultureInfo.InvariantCulture, "POINT({1} {0})", jObject[LATITUDE_KEY], jObject[LONGITUDE_KEY]);
    return DbGeography.FromText(wkt, DbGeography.DefaultCoordinateSystemId);
  }

  public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
  {
    var dbGeography = value as DbGeography;

    serializer.Serialize(writer, dbGeography == null || dbGeography.IsEmpty ? null : new { latitude = dbGeography.Latitude.Value, longitude = dbGeography.Longitude.Value });
  }
}

      

        
        
        
      

    

Then in my controller class (CentreController) I have the following lines:

public async Task<JsonResult> Get(int Id)
{
  var centre = new Centre {CentreId = Id};
  // This is a call to our data service, which basically loads the centre object
  // from the database via Entity Framework.
  centre = (await CentreService.Get(centre)).Data;

  //centre.Location = null;
  //var jc = JsonConvert.SerializeObject(centre);

  return Json(centre,JsonRequestBehavior.AllowGet);
}

      

        
        
        
      

    

This gives me a 500 error and an exception from Null values.

If I uncomment the line

//centre.Location = null;

      

        
        
        
      

    

then the code works fine, but no location is returned back to the caller.

If I uncomment the line

//var jc = JsonConvert.SerializeObject(centre);

      

        
        
        
      

    

Then my DbGeographyConverter is called and the jc object contains the correct data that I want to return, but in a string instead of a JsonResult. The Json call doesn't call the DbGeographyConverter and I can't figure out why.

How can I get a Json call to run my custom converter? Or, alternatively, how can I get my controller method to convert the JsonConvert output to JsonResult?