No datatype validation for indexed column on UPDATE

MariaDB [test]> CREATE TABLE `tc4091` (

    ->     `int_1` int,

    ->     `int_2` int,

    ->     KEY (`int_1`)

    ->     );

Query OK, 0 rows affected (0.017 sec)

MariaDB [test]>     INSERT INTO tc4091 (int_1,int_2) values (1,1);

Query OK, 1 row affected (0.001 sec)

MariaDB [test]>     UPDATE tc4091 SET int_2 = 4 WHERE int_1 = 'a';

Query OK, 0 rows affected (0.000 sec)

Rows matched: 0  Changed: 0  Warnings: 0

MariaDB [test]>     UPDATE tc4091 SET int_2 = 4 WHERE int_2 = 'a';

ERROR 1292 (22007): Truncated incorrect DOUBLE value: 'a'

MariaDB [test]>     DROP TABLE tc4091;

Query OK, 0 rows affected (0.007 sec)