Opaque not working as documented

Hi, I'm having an odd problem with CREATE FUNCTION. Here are my statements:

CREATE FUNCTION ENCODE_BURY_TYPE (OPAQUE) RETURNS INT
AS 'SELECT CASE
WHEN $1 = \'EXACT\' THEN 1
WHEN $1 = \'APPROX\' THEN 2
WHEN $1 = \'PSUEDO\' THEN 3
ELSE NULL
END;'
LANGUAGE 'SQL';

CREATE FUNCTION DECODE_BURY_TYPE (INT) RETURNS OPAQUE
AS 'SELECT CASE
WHEN $1 = 1 THEN \'EXACT\'
WHEN $1 = 2 THEN \'APPROX\'
WHEN $1 = 3 THEN \'PSUEDO\'
ELSE \'NONE\'
END;'
LANGUAGE 'SQL';

CREATE TYPE BURY_TYPE (
INPUT = ENCODE_BURY_TYPE,
OUTPUT = DECODE_BURY_TYPE,
INTERNALLENGTH = 16
);

This results in the error messages

ERROR: SQL functions cannot have arguments of type "opaque"
ERROR: SQL functions cannot return type "opaque"

Changing opaque to text results in the CREATE FUNCTION statements
working, while the CREATE TYPE statements throw a fit.

I suspect a configuration problem, but I have no idea where to look and
searches on google and the list archives haven't turned up anything. I'm
running the redhat rpms for Postgresql 7.2.1 on redhat 7.3.

Thank you,

Steve