my sql function returm multipal value please solve ?

Hi, All

when i execute  this function, this  showing error :- 

Error Code : 1242
Subquery returns more than 1 row

Actually i want to select particular month No. I have date like 01-Aug-2013 (dd-mmm-yyyy)and i want to convert this into (yymmdd) format than after call this function in sp 

DELIMITER $$

DROP FUNCTION IF EXISTS `pnb_etl`.`fn_monthname`$$

CREATE DEFINER=`loyetl`@`localhost` FUNCTION `fn_monthname`(v_monthNam varchar(3)) RETURNS int(2)
BEGIN
 DECLARE  monthNo int(2) DEFAULT null;
if v_monthNam =(select (SUBSTRING(BSN_DATE,4,3)) from loylty_recon_intermediate)then
   CASE 
   WHEN v_monthNam='jan' THEN
             SET monthNo = 01;
   WHEN v_monthNam='feb' THEN
             SET monthNo = 02;
   WHEN v_monthNam='mar' THEN
             SET monthNo = 03;
   WHEN v_monthNam='apr' THEN
             SET monthNo = 04;
   WHEN v_monthNam='may' THEN
             SET monthNo = 05;
   WHEN v_monthNam='jun' THEN
             SET monthNo = 06;
   WHEN v_monthNam='jul' THEN
             SET monthNo = 07;
   WHEN v_monthNam='aug' THEN
             SET monthNo = 08;
   WHEN v_monthNam='sep' THEN
             SET monthNo = 09;
   WHEN v_monthNam='oct' THEN
             SET monthNo = 10;
   WHEN v_monthNam='nov' THEN
             SET monthNo = 11;
   WHEN v_monthNam='dec' THEN
             SET monthNo = 12;
   ELSE
      SET monthNo=0;
   END CASE;
    RETURN monthNo;
end if;
    END$$

DELIMITER ; 

Please guide me where is the problem in my coding.

Thanks,

Dhiraj