为什么我不能在 SYS 拥有的对象上创建触发器?

While trying to create a trigger named ghazal_current_bef_upd_row :

create trigger ghazal_current_bef_upd_row
before update on ghazal_current
for each row 
when (new.Rating < old.Rating)
begin

insert into ghazal_current_audit
 (GhazalName,Old_Rating,New_Rating)
 values
 (:old.GhazalName,:old.Rating,:new.Rating);
end;

我收到以下错误:

Error report:
ORA-04089: cannot create triggers on objects owned by SYS
04089. 00000 -  "cannot create triggers on objects owned by SYS"
*Cause:    An attempt was made to create a trigger on an object owned by SYS.
*Action:   Do not create triggers on objects owned by SYS.

名为ghazals_current 和ghazal_current_audit 的表均由SYS 创建.为什么我不能在 SYS 创建的表上创建触发器.