Trigger

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();

If I do an update with a tag_number that does not exist everything is fine.
If  I do an update with a preexisting number I get the following messages-
NOTICE: plpgsql:ERROR during compile of validate_tag_number line 3
ERROR: unterminated " in name "
I was expecting to get the "number is already present" error messsage.
Where is my error?

    Thank you
    Adrian Klaver