PL/pgSQL triggers ON INSERT OR UPDATE

Hello, I'm trying to set up a trigger on insert or update
but when using the predefined variable ``OLD'' I get a
NOTICE from the trigger function about OLD not being defined yet.

Of course OLD is not defined when the function is triggered on INSERT
event, and I did not mention it if not inside a conditional block
checking for the TG_OP variable being 'UPDATE'.

For better understanding here is some code:

BEGIN
    IF TG_OP = 'UPDATE' THEN
        IF OLD.id <> NEW.id THEN
            -- do the work
        END IF
    END IF;
END;

Even when TG_OP != 'UPDATE' (INSERT event) I still get an error
message from the pl/pgsql compiler (the first time the trigger is fired).

What should I do then ? Is it still possible to use the same function
for UPDATE OR INSERT events ?

TIA

--san;