In a map, I forgot to set the attribute to <empty> and therefore could not assign a value to it after the map completed.
InfoMsg.MessagePart.FileName=System.IO.Path.GetFileName(TransmissionMsg(FILE.ReceivedFileName));
InfoMsg.MessagePart.Occurrence=System.Convert.ToString(ThisInterchange);
However the error that is displayed in the ‘Service Details’ is somewhat misleading:
Where is the XPATH expression that does not have a node?