Error on natural join with Subquery alias

[FlyingMao]

steps:

mysql> CREATE TABLE a_test(`id` int(10) unsigned NOT NULL,`t_id` int(10) unsigned NOT NULL DEFAULT '0',`name` char(120) NOT NULL DEFAULT '',`pad` int(11) NOT NULL,PRIMARY KEY (`id`),KEY `k_1` (`t_id`));
Query OK, 0 rows affected (0.16 sec)

mysql> CREATE TABLE a_order(`id` int(10) unsigned NOT NULL,`o_id` int(10) unsigned NOT NULL DEFAULT '0',`name` char(120) NOT NULL DEFAULT '',`pad` int(11) NOT NULL,PRIMARY KEY (`id`),KEY `k_1` (`o_id`));
Query OK, 0 rows affected (0.09 sec)

mysql> insert into a_test values(1,1,'test中id为1',1),(2,2,'test_2',2),(3,3,'test中id为3',4),(4,4,'$test$4',3),(5,5,'test...5',1),(6,6,'test6',6);
Query OK, 6 rows affected (0.02 sec)

mysql> insert into a_order values(1,1,'order中id为1',1),(2,2,'test_2',2),(3,3,'order中id为3',3),(4,4,'$order$4',4),(5,5,'order...5',1);
Query OK, 5 rows affected (0.01 sec)

mysql> select * from a_test a natural join (select * from a_order where pad>2) b ;
ERROR 1003 (HY000): Unknown column 'b.o_id' in 'field list'