oracle id 自增 报错 ORA-04079: 无效的触发器说明

在行 1 上开始执行命令时出错:
create or replace trigger Lidtri
before insert on LEAF SPEC
for each row
begin
select Lidtri.nextval into :new.LID from dual;
end;
错误报告:
ORA-04079: 无效的触发器说明
04079. 00000 - "invalid trigger specification"
*Cause: The create TRIGGER statement is invalid.
*Action: Check the statement for correct syntax.