BEFORE INSERT triggers dont issue 1406 error

MariaDB [test]> USE test;

Database changed

MariaDB [test]> SET @@session.sql_mode = 'STRICT_ALL_TABLES,STRICT_TRANS_TABLES'

;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> DROP TABLE IF EXISTS t1;

Query OK, 0 rows affected (0.06 sec)

MariaDB [test]> CREATE TABLE t1 (

    ->  c CHAR(1) NOT NULL

    -> ) ENGINE = InnoDB;

Query OK, 0 rows affected (0.08 sec)

MariaDB [test]> DELIMITER ||

MariaDB [test]> CREATE TRIGGER t1_bi

    ->  BEFORE INSERT

    ->  ON t1

    ->  FOR EACH ROW

    -> BEGIN

    ->  SET NEW.c = 'www';

    -> END;

    -> ||

Query OK, 0 rows affected (0.04 sec)

MariaDB [test]> DELIMITER ;

MariaDB [test]> INSERT INTO t1 VALUES ('a');

Query OK, 1 row affected (0.00 sec)

Records: 1  Duplicates: 0  Warnings: 0

MariaDB [test]> SHOW WARNINGS;

Empty set (0.00 sec)

MariaDB [test]> SELECT * FROM t1;

+---+

| c |

+---+

| w |

+---+

1 row in set (0.00 sec)