Trigger once again

I have created the following function:

CREATE OR REPLACE FUNCTION validate_tag_number()
RETURNS OPAQUE AS '
BEGIN
IF new.tag_number=old.tag_number THEN
RAISE EXCEPTION ''Number is already present '';
END IF;
RETURN NEW;
END;
'LANGUAGE 'plpgsql';

I used it to create a trigger as follows;

CREATE TRIGGER validate_tag_number BEFORE UPDATE ON tag_vendor FOR EACH ROW
EXECUTE PROCEDURE validate_tag_number();

The trigger seems to work to well. If I update values other than the
tag_number I get the 'Number is already present' message. I am somewhat
confused as their is no new.tag_number value being passed to the function.
Can anyone set me straight?

Thank you
Adrain Klaver