DB2 Runtime Error

Question

UPDATE
      S
SET

    (S.a,
    S.b,
    S.c)
=
(

SELECT 
    FS.a,
    FS.b,
    FS.c
FROM

(SELECT 
    T.a,   
    T.b,    
    T.c,     
    T.d,   
    T.e,    
    CASE 
     WHEN IS.a IS NULL THEN 'N'
     ELSE 'Y'
    END 

FROM 
    t       T
        JOIN
           y
          ON(y.a = T.a)




) FS


WHERE
    FS.a    = S.e
    AND FS.b    = S.a
    AND FS.c    = S.b
    AND FS.d    = S.c
)

While i execute this query iam getting an error :

 21:23:59  [UPDATE - 0 row(s), 0.000 secs]  [Error Code: -811, SQL State: 21000]  DB2 SQL Error: SQLCODE=-811, SQLSTATE=21000, SQLERRMC=null, DRIVER=3.50.152
... 1 statement(s) executed, 0 row(s) affected, exec/fetch time: 0.000/0.000 sec  [0 successful, 0 warnings, 1 errors]

Can u please explain me the root cause...?