a little doubr about domains and pl/python

Hi all. I'm playing with pl/python AND i'm with a doubt.

I wanna create a domain to roman numerals.

I have made:

CREATE OR REPLACE FUNCTION valid_roman(text)
RETURNS BOOLEAN
IMMUTABLE
LANGUAGE plpythonu AS
$$
from roman9 import fromRoman

roman = args[0]
try:  r = fromRoman(roman)  return 'true'
except:   return 'false'
$$;

CREATE DOMAIN romano AS text CHECK (valid_roman(VALUE)) ;


CREATE OR REPLACE FUNCTION to_roman(integer)
RETURNS romano
IMMUTABLE
LANGUAGE plpythonu AS
$$
from roman9 import toRoman

number = args[0]
try:  r = toRoman(number)  return r
except: return 'false'
$$;

CREATE CAST (integer AS romano) WITH FUNCTION to_roman(integer);

And that works:

SELECT to_roman(50)
SELECT to_roman(100)
SELECT valid_roman('XX')
SELECT valid_roman('XXX')
SELECT valid_roman('XXXX')

But my cast doen't works:

SELECT 5::romano

ERROR:  value for domain romano violates check constraint "romano_check"

What's wrong?

[]'s
- Walter