Fast index creation fails with DROP/ADD index

show create table sbtest1;
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| sbtest1 | CREATE TABLE `sbtest1` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `k` int(10) unsigned NOT NULL DEFAULT '0',
  `c` char(120) NOT NULL DEFAULT '',
  `pad` char(60) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`),
  KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=30001 DEFAULT CHARSET=latin1 MAX_ROWS=1000000 |
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select count(*) from sbtest1;
+----------+
| count(*) |
+----------+
| 30000 |
+----------+
1 row in set (0.51 sec)

mysql> alter table sbtest1 drop index k_1, add index k_1 (k);
Query OK, 30000 rows affected (1.38 sec)
Records: 30000 Duplicates: 0 Warnings: 0

However, following works fine:

mysql> alter table sbtest1 drop index k_1, add index k_2 (k);
Query OK, 0 rows affected (0.61 sec)
Records: 0 Duplicates: 0 Warnings: 0