Ignorable index makes the resulting CREATE TABLE invalid

MariaDB [test]> CREATE TABLE t (a INT, KEY (a) IGNORE);

Query OK, 0 rows affected (0.046 sec)

MariaDB [test]> SHOW CREATE TABLE t;

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

| Table | Create Table                                                                                                  |

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

| t     | CREATE TABLE `t` (

  `a` int(11) DEFAULT NULL,

  KEY `a` (`a`) IGNORED

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.001 sec)

MariaDB [test]> DROP TABLE t;

Query OK, 0 rows affected (0.031 sec)

MariaDB [test]> CREATE TABLE `t` (

    ->   `a` int(11) DEFAULT NULL,

    ->   KEY `a` (`a`) IGNORED

    -> ) ENGINE=InnoDB DEFAULT CHARSET=latin1 ;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ') ENGINE=InnoDB DEFAULT CHARSET=latin1' at line 4