FaultException on RetrieveMultipleRequest for Entity lookupmapping

Hi,

I am trying to use a FetchXML with a RetrieveMultipleRequest/RetrieveMultipleResponse for entity "lookupmapping", and am getting an unexpected exception from the online version of CRM 2011:

Timestamp: '2/10/2013 4:15:02 PM'.  Error Code: '-2147220970'. Message: 'An unexpected error occurred.'.  Inner Fault Message: 'An unexpected error occurred.'...

Code snippet:

            ParseOneException pex = null;
            RetrieveMultipleRequest request = null;
            FetchExpression fetchExpression = null;
            RetrieveMultipleResponse response = null;

            try
            {
                request = new RetrieveMultipleRequest();
                fetchExpression = new FetchExpression(inFetchXML);
                request.Query = fetchExpression;

                response = (RetrieveMultipleResponse)inService.Execute(request);  // Exception is thrown here
            }
            catch (FaultException<OrganizationServiceFault> ex)
            {

                 // exception is caught here

            }

Is it not possible to get this entity using FetchXML like this ?  I have a few other entities (eg calendarrule) where the error message is quite specific - not supported by RetrieveMultiple.  But this is an "unexpected error".

The CRM is online CRM 2011.

Thanks and regards

Keith Murphy