Error: could not find pathkey item to sort

Hi Everyone,
          I had some problem with postgresql 8.3.1. I had got an error 
message, ERROR:  could not find pathkey item to sort, when
I use this query statement:
          SELECT   INTO   wk_fcstrec_dt   DISTINCT  MAX(fcstrec_dt) 
FROM a_cusfcst                            WHERE                  cscd = '2203'              AND    cs_partno =
'35732SSA'             AND    delipnt_no = '001'              AND    plantcd_no = '003'              AND    locatto =
trim(bothFROM 'SS')              AND    fcstrec_dt < '20080910'              AND    latest_flg = 'N';
 

      I have not ever seen the error in version 8.1 and 8.2. Now, I 
have already revised my program by removed the word "DISTINCT" but I 
want to know what is cause of this error. Could you please to explain to me?

Thank you very much.