创建关键字自动增加的触发器出现问题了，求帮助

SQL code

create sequence customerid increment by 1 start with 1 maxvalue 999999999

create or replace trigger bef_ins_customer_autoid
before insert on customer 
referencing new as newrow
for each row
when(newrow.id is null)
begin
  select customerid.nextval into :newrow.id  from dual
end;

但是编译时有warning，而且真正插入语句时也无法插入，
SQL Error: ORA-04098: 触发器 'bef_ins_customer_autoid' 无效且未通过重新验证
04098. 00000 - "trigger '%s.%s' is invalid and failed re-validation"
*Cause: A trigger was attempted to be retrieved for execution and was
  found to be invalid. This also means that compilation/authorization
  failed for the trigger.
*Action: Options are to resolve the compilation/authorization errors,
  disable the trigger, or drop the trigger

实测创建触发器
SQL code

CREATE OR REPLACE TRIGGER bef_ins_customer_autoid
BEFORE INSERT ON Customer 
REFERENCING NEW AS newrow
FOR EACH ROW
WHEN (newrow.id IS NULL)
BEGIN
  SELECT customerid.nextval into :newrow.id  FROM dual;
END;

实测结果：

学习了

嗯？这是什么情况？为什么我这边执行SQL code

    insert into Customer(id,name,phonenum,address)
    values (null,'xiaowang','1412341234','beijing') 

总是出现错误呢？我用的oracle sqldeveloper 会不会和这个有关系？但是不应该啊？