BUG #2847: Bug with IN statement

The following bug has been logged online:

Bug reference:      2847
Logged by:          Sandip
Email address:      sandip@singapore.com
PostgreSQL version: 8.1
Operating system:   Windows XP
Description:        Bug with IN statement
Details:

this is my function:

CREATE OR REPLACE FUNCTION sp_get_phase(character varying, character
varying, character varying)
  RETURNS ret_dv_sp_get_phase AS
$BODY$
SELECT          BOOK_NO, USER_ID, COMPANY_ID, PHASE, UPDATE_USER_ID,
UPDATE_DATE,
                      AddInfo1, AddInfo2
FROM            T_PHASE
WHERE           (USER_ID = $1) AND (COMPANY_ID = $2) AND BOOK_NO IN ($3)
$BODY$
  LANGUAGE 'sql' VOLATILE;


When I run
select * from sp_get_phase ('sandip', 'oms', '4')   returns 1
record.....this works fine....

select * from sp_get_phase ('sandip', 'oms', '1')  returns 1 record.....this
also works fine... BUT

select * from sp_get_phase ('sandip', 'oms', '1,4') this return a Blank
record.

I tried to execute the SQL statement from the function

SELECT          BOOK_NO, USER_ID, COMPANY_ID, PHASE, UPDATE_USER_ID,
UPDATE_DATE,
                      AddInfo1, AddInfo2
FROM            T_PHASE
WHERE           (USER_ID = 'testUser') AND (COMPANY_ID = 'testCompany') AND
BOOK_NO IN (1,4)

----- This Works fine... returns 2 records.   What may be the problem?

Thanks in advance.
Regards,
Sandip.