ALTER TABLE .. ADD UNIQUE INDEX IF NOT EXISTS causes syntax error

create table t1 (i int);

alter table t1 add unique index if not exists idx(i);

MariaDB [test]> create table t1 (i int);

Query OK, 0 rows affected (0.32 sec)

MariaDB [test]> alter table t1 add unique index if not exists idx(i);

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 'if not exists idx(i)' at line 1

MariaDB [test]> create unique index if not exists idx2 on t1 (i);

Query OK, 0 rows affected (0.42 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table t1 add unique index idx1(i);

Query OK, 0 rows affected (0.33 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table t2 add fulltext index if not exists idx(b);

Query OK, 0 rows affected (0.23 sec)               

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> alter table t2 add spatial index if not exists idx2(a);

Query OK, 0 rows affected (0.19 sec)               

Records: 0  Duplicates: 0  Warnings: 0

revision-id: bar@mariadb.org-20130424142022-u4xhikvoqggze9b0

revno: 3745

branch-nick: 10.0