QueryByCriteria#addGroupBy question

[Hiller, Frank RD-AS2]

Hi,
 
I'm using ojb1.0.3 and Oracle 10g.
I tried to use QueryByCriteria#addGroupBy but I get the error
"java.sql.SQLException: ORA-00979: not a GROUP BY expression"
 
What's the way to retrieve a collection of objects, filtered with the
"group by" attribute?
 
Thanks,
Frank