为什么在触发器里编译不了Case语句啊。

Create Or Replace Trigger After_insert_Receive
  after insert on TB_SMS_RECEIVE
  REFERENCING OLD AS old_v NEW AS new_v FOR EACH ROW
declare
  ID Number;
begin
  ID:=1;
  update TB_SMS_SURVEY
  set
    AWRESULT=(case when '2'='2' then '2' else '1' end)
  where
    TB_SMS_SURVEY.Id=ID;
end ;
编译不通过啊，但是把里面的语句拿出来单独执行没有问题啊。