Sybase Philippines

i am using ASA 9.0 and i have a very simple query like this:

SELECT LASTNAME, FIRSTNAME, MATERNALNAME
FROM EMPLOYEE
GROUP BY LASTNAME

but evrytime i run this query its throwing error:
Function or column reference to FIRSTNAME must also appear in the GROUP BY


as far as i know theres nothing wrong with this query, because i have been using this kind of query in many databases, as well as ASE.
Can anyone help me how can i resolved it?

Thanks in advance...