OldValue and NewValue attributes do not show up in queries on CaseHistory

[metadaddy]

I am using the following query with the SforcePartnerClient:

SELECT Id, CaseId, Field, CreatedById, CreatedDate, IsDeleted, NewValue, OldValue FROM CaseHistory WHERE CaseId='SOME_VALID_CASE_ID' 

I get values for all the requested fields, except NewValue and OldValue - they are not in the response. Looking at the SOAP traffic, I do see the OldValue and NewValue fields on the wire.