What is wrong with this code, getting ORA-920

[MrT]

CREATE OR REPLACE TRIGGER SDS_TRANS_FISCAL_YR_TRIG
 BEFORE INSERT
 ON SDS_TRANSACTION_TABLE
 FOR EACH ROW
 WHEN (:new.FISCAL_YEAR IS NULL)
-- PL/SQL Block
BEGIN
 IF TO_DATE(:new.transaction_date,'MON/DD/YYYY') > TO_DATE('AUG/31/'||TO_CHAR(sysdate,'YYYY'),'MON/DD/YYYY')  THEN
   :new.fiscal_year := 'FY'||SUBSTR(TO_CHAR(sysdate,'YYYY'),3)||'-'||SUBSTR(TO_CHAR(TO_NUMBER(TO_CHAR(sysdate,'YYYY'))+1),3);
 ELSE
   :new.fiscal_year := 'FY'||SUBSTR(TO_CHAR(TO_NUMBER(TO_CHAR(sysdate,'YYYY'))-1),3)||'-'||SUBSTR(TO_CHAR(sysdate,'YYYY'),3);
 END IF;

END;