ERROR: SELECT DISTINCT ON with postgresql v 7.1.2

Hello,

I have a problem white one sql request. I got this error message :

Warning: PostgreSQL query failed: ERROR: SELECT DISTINCT ON expressions
must match initial ORDER BY expressions in
/export/castor-b7/local-home/kelbertj/Prog/web/lumiere/admin/recherche_realisateurs.php
on line 85 ERROR: SELECT DISTINCT ON expressions must match initial
ORDER BY expressions SELECT DISTINCT ON (people_id)
people_id,people_lastname,people_firstname from people where
lower(people_firstname) ~* (SELECT text_accents('\\\"Luc\\$')) order by
people_lastname ASC limit 40 offset 0

I didn't find any solution to this problem ! If you have any idea I'll
be most gratefull If you could answer !

Thanks

-- 
Jean-Michel Kelbert