在WCF http服务中返回自定义错误消息

我刚开始学习C#，有点拘泥于此。

当用户没有访问特定资源的权限时，我需要返回一条自定义错误消息。

服务接口类别：

[ServiceContract]
public interface IPatientDemographics
{
    [OperationContract]   
    [WebInvoke(Method = "GET", UriTemplate = "GetPatientDemographics/{strHospId}/{strView=mobile}",
    ResponseFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.Wrapped)]
    [return: MessageParameter(Name = "PatientDemographicsResultSet")]
    List<PatientDemographics> GetPatientDemographics(string strHospId, string strView);
    [OperationContract]
    [FaultContract(typeof(AccessError))]
    [return: MessageParameter(Name = "Error")]
    AccessError ReturnAccessError();        
}
[DataContract]
public class AccessError
{
    [DataMember]
    public bool Error { get; set; }
    [DataMember]
    public string Message { get; set; }
}

服务类别：

[ServiceBehavior(AddressFilterMode=AddressFilterMode.Any)]
public class PatientDemographics : IPatientDemographics
{
    public List<PatientDemographics> GetPatientDemographics(string strHospId, string strView)
    {
        AuthenticationDAO authenticationDAO = new AuthenticationDAO();
        string userName = GetUserNameFromHeader();
        if (authenticationDAO.CheckPatientSealAccess(strHospId, userName, "-1", "N"))
        {
            PatientDAO patientDAO = new PatientDAO();
            patientDAO.AuditPatient(strHospId, userName);
            return patientDAO.GetPatientDemographics(strHospId);
        }
        else
        {
            AccessError serviceData = new AccessError();
            serviceData.Error = true;
            serviceData.Message = "User does not have access to patient record";
            throw new FaultException<AccessError>(serviceData, "User does not have access to patient record");
        }
    }
}

目前，如果用户没有访问权限，这只会返回一个null对象。我试着抛出FaultException，但没有成功。理想情况下，我希望它以JSON格式返回，比如：

{
    error: "User does not have access to patient record"
}