得到& # 39;Microsoft.SharePoint.SoapServer.SoapServerException

下面的代码给出了'Microsoft.SharePoint.SoapServer。SoapServerException '

    public class SP_Access
    {
        ListsSoapClient _proxy;
        XElement _lists; 
        public SP_Access()
        {
            _proxy = new ListsSoapClient();
            //authenticcation
            _proxy.ClientCredentials.Windows.ClientCredential = new System.Net.NetworkCredential();
            //Define the parameters for the service call
            XElement query = new XElement("Query");
            XElement queryOptions = new XElement("QueryOptions");
            XElement viewFields = new XElement("ViewFields",
                                        new XElement("FieldRef", new XAttribute("Name", "Employer")),
                                        new XElement("FieldRef", new XAttribute("Name", "Address")));
            _lists = _proxy.GetListItems("Search Files",
                    null,
                    query,
                    viewFields,
                    null,
                    queryOptions,
                    null);
        }
    }

是任何错误的代码，或者我应该改变任何配置。在客户端或服务器端。谢谢。