ORA-01427- Single row subquery returns more then 1 row

Hi

I have following query which is giving  me the error ORA-01427- Single row subquery  returns more then 1 row. When I omit the bold condition in where clause i get the data without any error but when i add that condition i am getting above error. Any help Please.

SELECT
  B.PTD
  ,b.LBS
  FROM
  (
  SELECT       A.SEGMENT2,
               A.LOBS LBS,
               A.PERIOD_NAME,
               A.PERIOD_YEAR,
               NVL(SUM(A.ACCOUNTED_DR), 0) - NVL(SUM(A.ACCOUNTED_CR), 0) PTD
          FROM (SELECT C.SEGMENT2,
                       DECODE((select cc.segment7
                                from xla_ae_lines l, gl_code_combinations cc,xla_ae_headers h