Newbie: execute function error!

I created a sample function:

CREATE FUNCTION concat_text (text, text) RETURNS text AS '
BEGIN
RETURN $1 || $2;
END; '
LANGUAGE 'plpgsql';

The creation was fine. But when I execute the function with : select
concat('a', 'b');

I get the error:

NOTICE: plpgsql: ERROR during compile of concat_text near line 1
"RROR: parse error at or near "

What did I do wrong?

Thanks!