not a GROUP BY expression??

Why am I get this error message when I execute this query?

SELECT ODisciplina.BOUI FROM ODisciplina , Disciplina$student
WHERE ((Disciplina$student.PARENT$(+)=ODisciplina.BOUI))
GROUP BY (ODisciplina.BOUI )HAVING( Count(CHILD$)=0 )
ORDER BY (ODisciplina.NAME)

without the 3rd line it works and without the last line it works too.Tks, I've found the error. It should be:

SELECT ODisciplina.BOUI FROM ODisciplina , Disciplina$student
WHERE ((Disciplina$student.PARENT$(+)=ODisciplina.BOUI))
GROUP BY (ODisciplina.BOUI, ODisciplina.NAME )HAVING( Count(CHILD$)=0 )
ORDER BY (ODisciplina.NAME)