trigger wont use internal function?

I am trying to use the following trigger:

CREATE TRIGGER formatname BEFORE update OR insert ON mytable FOR EACH row
EXECUTE PROCEDURE lower(name);

however, it gives me the message:

ERROR: CreateTrigger: function lower() does not exist

obviously this does exist, since I can do inserts/updates/selects using
lower(). I have also tried creating my own version of a lower function but
it gives me the same message.

Am I missing something? This seems like it should be pretty straightforward.
tia,

robert