This is the exception

System.Web.Services.Protocols.SoapException: Server was unable to process request. ---> System.InvalidOperationException: There was an error generating the XML document. ---> System.InvalidOperationException: Cannot serialize the DataTable. DataTable name is not set.
   at System.Data.DataTable.WriteXmlSchema(XmlWriter writer, Boolean writeHierarchy)
   at System.Data.DataTable.System.Xml.Serialization.IXmlSerializable.WriteXml(XmlWriter writer)
   at System.Xml.Serialization.XmlSerializationWriter.WriteSerializable(IXmlSerializable serializable, String name, String ns, Boolean isNullable, Boolean wrapped)
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write3_AddMessageResponse(Object[] p)
   at Microsoft.Xml.Serialization.GeneratedAssembly.ArrayOfObjectSerializer5.Serialize(Object objectToSerialize, XmlSerializationWriter writer)
   at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
   --- End of inner exception stack trace ---
   at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
   at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle)
   at System.Web.Services.Protocols.SoapServerProtocol.WriteReturns(Object[] returnValues, Stream outputStream)
   at System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[] returnValues)
   at System.Web.Services.Protocols.WebServiceHandler.Invoke()
   --- End of inner exception stack trace ---

[WebMethod]
         public DataTable AddMessage(string sDealer, string sUser, string sMsg, DataTable oDt)
        {
            if (oDt == null)
            {
                oDt = new DataTable();
                oDt.Columns.Add(new DataColumn("UserName", typeof(string)));
                oDt.Columns.Add(new DataColumn("Message", typeof(string)));
                oDt.Columns.Add(new DataColumn("Date", typeof(string)));
            }
            DataRow oDr = oDt.NewRow();
            oDr["UserName"] = sUser;
            oDr["Message"] = sMsg;
            oDr["Date"] = DateTime.Now.ToString();
            oDt.Rows.Add(oDr);

            string sTime = DateTime.Now.ToString();
            string sAddText = sDealer + "~" + sUser + "~" + sMsg + "~" + sTime;
            return oDt;
        }