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