[MySQL] Problema con REFERENCING in un trigger

[Alhazred]

Per quale motivo questo trigger non viene accettato da MySQL 5.1?
Mi da un errore di sintassi 'REFERENCING NEW ROW AS new_user, OLD ROW AS old_user
FOR EACH ROW
ModificaUserna' at line 3

CREATE TRIGGER modifica_username
AFTER UPDATE ON username_view
REFERENCING NEW ROW AS new_user, OLD ROW AS old_user
FOR EACH ROW
ModificaUsername(old_user.username_view, new_user.username_view);