MyISAM/Aria table is marked as crashed upon updating NULL values in DESC index

create table t1 (c char(10), key(c desc)) engine=MyISAM;

insert into t1 values (null),(null);

update t1 set c='foo';

MariaDB [test]> create table t1 (c char(10), key(c desc)) engine=MyISAM;

Query OK, 0 rows affected (0.015 sec)

MariaDB [test]> insert into t1 values (null),(null);

Query OK, 2 rows affected (0.001 sec)

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> update t1 set c='foo';

ERROR 126 (HY000): Index for table './test/t1.MYI' is corrupt; try to repair it

MariaDB [test]> check table t1;

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

| Table   | Op    | Msg_type | Msg_text                                                |

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

| test.t1 | check | warning  | Table is marked as crashed                              |

| test.t1 | check | error    | Checksum for key:  1 doesn't match checksum for records |

| test.t1 | check | error    | Corrupt                                                 |

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

3 rows in set (0.001 sec)