Casting

I'm obviously missing something here but I don't know what. This is on 7.3.2.
When I use my cast I get:

mydb=> select cast('truE'::text as boolean);
WARNING: Error occurred while executing PL/pgSQL function text_to_bool
WARNING: line 4 at select into variables
ERROR: CopyQuerySnapshot: no snapshot has been set

Any clues as to why that is?

The code is simplistic to say the least:

-- text_to_bool ()
--
-- Cast from text type to boolean.
--
CREATE OR REPLACE FUNCTION text_to_bool ( text )
RETURNS boolean
AS '
DECLARE
rv boolean;
BEGIN
SELECT INTO rv
CASE
WHEN lower($1) = ''true''
OR lower($1) = ''t''
OR lower($1) = ''1''
THEN
true
WHEN lower($1) = ''false''
OR lower($1) = ''f''
OR lower($1) = ''0''
THEN
false
ELSE
NULL
END;
RETURN rv;
END;
'
LANGUAGE 'plpgsql'
STRICT
IMMUTABLE;

CREATE CAST (text AS boolean) WITH FUNCTION text_to_bool ( text );

Thanks,

--
Nigel J. Andrews