alter table - failing to ADD KEY IF NOT EXISTS when existing dropping the same index name earlier in the alter table

> CREATE TABLE b (`transaction_id` int(11) NOT NULL DEFAULT '0', KEY `transaction_id` (`transaction_id`));

Query OK, 0 rows affected (0.01 sec)

> alter table b DROP KEY transaction_id, ADD UNIQUE KEY IF NOT EXISTS (transaction_id);

Query OK, 0 rows affected, 1 warning (0.00 sec)

Records: 0  Duplicates: 0  Warnings: 1

> show create table b;

+-------+-----------------------------------------------------------------------------------------------------------+

| Table | Create Table                                                                                              |

+-------+-----------------------------------------------------------------------------------------------------------+

| b     | CREATE TABLE `b` (

  `transaction_id` int(11) NOT NULL DEFAULT '0'

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

+-------+-----------------------------------------------------------------------------------------------------------+