При POST запросе получаю null

Метод

[HttpPost]
public HttpResponseMessage Post([FromBody]XMLRequests xmlRequest)

при получении запроса xmlRequest всегда null. Не могу понять в чем проблема

 [HttpPost]
    public HttpResponseMessage Post([FromBody]XMLRequests xmlRequest)//HttpResponseMessage
    {
        HttpResponseMessage response;
        if (xmlRequest != null)
        {
            response = Request.CreateResponse(HttpStatusCode.OK);
        }
        else
        {response = Request.CreateResponse(HttpStatusCode.BadRequest);}
    }
 public class XMLRequests
{
    [XmlElement("action")]
    public string action { get; set; }
    [XmlElement("msg_id")]
    public string msg_id { get; set; }
    [XmlElement("a_number")]
    public string a_number { get; set; }
    [XmlElement("b_number")]
    public string b_number { get; set; }
    [XmlElement("call_datetime")]
    public string call_datetime { get; set; }
}