wcf rest json return collection

I am attempting to build a Restful WCF Service which returns data in JSON format. My firsts methods work fine but when I try return a collection my test program receive the next exception:

Unable to write data to the transport connection. An existing connection was forcibly closed by the remote host.

My Service code:

[ServiceContract]
public interface IService
{ 
  [OperationContract]
    [WebInvoke(Method = "POST", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.Bare, UriTemplate = "/GetModes")]
    OGetModesResponse OGetModes(OGetModesRequest oGetModes);
}
[DataContract]
public class OGetModesRequest
{
    private String m_sTicket;

    [DataMember]
    public String prTicket
    {
        get { return m_sTicket; }
        set { m_sTicket = value; }
    }
}

[DataContract]
public class OGetModesResponse
{
    [DataMember]
    public string sTicket;
    [DataMember]
    public emStatus emStatus;
    [DataMember]
    public IList<CTMode> aoModes;
}

And my test program:

OGetModesRequest oGetModes = new OGetModesRequest { prTicket = sTicket };
ser = new DataContractJsonSerializer(typeof(OGetModesRequest));
mem = new MemoryStream();
ser.WriteObject(mem, oGetModes);
webClient = new WebClient();
webClient.Headers["Content-type"] = "application/json";
webClient.Encoding = Encoding.UTF8;
//Exception here
bData = webClient.UploadData("http://localhost:26104/Service.svc/GetModes", "POST", mem.ToArray()); 
stream = new MemoryStream(bData);
obj = new DataContractJsonSerializer(typeof(OGetModesResponse));
OGetModesResponse OResultModes = obj.ReadObject(stream) as OGetModesResponse;

I debug my services and works fine. What can be happening?

Thanks for help.

Edit (solution): CTMode is a class used by managing object that I obtain using NHibernate so I create a new class serializable called CMode

[DataContract]
public class OGetModesResponse
{
    [DataMember]
    public string sTicket;
    [DataMember]
    public emStatus emStatus;
    [DataMember]
    public IList<CMode> aoModes;
}
[Serializable]
public class CMode
{
    public Int32 nId;
    public Int32 nCode;
    public String sName;
}

Solution

Try to check inner exception and add some logging/ trace on the server.

There are few possibilities for your (generic) error as you may not be aware about inner exception:

object CTMode is missing DataContract, DataMember attribute.
object CTMode is an enum that missing attributes or has incorrect value that cannot be serialized
previous connection is not closed correctly
there is a proxy server on the way and you need to bypassed it