CHECK constraint fails on intermediate step of ALTER

MariaDB [test]> create table t1 (a int, check(a>0 or a is null));

Query OK, 0 rows affected (0.41 sec)

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

Query OK, 1 row affected (0.06 sec)

MariaDB [test]> alter table t1 modify a int auto_increment primary key;

ERROR 4025 (23000): CONSTRAINT `CONSTRAINT_1` failed for `test`.`#sql-4c9e_4`

MariaDB [test]> set check_constraint_checks=0;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> alter table t1 modify a int auto_increment primary key;

Query OK, 1 row affected (1.01 sec)

Records: 1  Duplicates: 0  Warnings: 0

MariaDB [test]> select * from t1;

+---+

| a |

+---+

| 1 |

+---+

1 row in set (0.00 sec)