Webservice not returning XML error

Hi Guys,

I am having trouble with a webservice and not sure what the problem is.

I am trying to call a large amount of XML from the webservice for use in my project.

When I try and retrieve it I get this error: NullReferenceException: Object reference not set to an instance of an object on this line sXML = client.GetWorldScenarioXml(sessID ,scenID);

 string sXML = " ";
                     Guid sessID = new Guid("CA761232-ED42-11CE-BACD-00AA0057B223");
                     Guid scenID = new Guid("e33898de-6302-4756-8f0c-5f6c5218e02e");
                     sXML = client.GetWorldScenarioXml(sessID ,scenID);

Any help would be much appreciated thanks.