Illegal mix of collations on timestamp

MariaDB [test]> set charset latin2;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> DELETE FROM  annonce  WHERE  `created_on` <= '2013-11-15 00:41:28' - INTERVAL 7 DAY;

ERROR 1267 (HY000): Illegal mix of collations (latin1_swedish_ci,NUMERIC) and (latin2_general_ci,NUMERIC) for operation '<='

MariaDB [test]> desc annonce;

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

| Field      | Type                   | Null | Key | Default           | Extra          |

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

| id         | mediumint(8) unsigned  | NO   | PRI | NULL              | auto_increment |

| users_id   | mediumint(8) unsigned  | NO   | MUL | NULL              |                

| note       | text                   | NO   |     | NULL              |                |

| created_on | timestamp              | NO   | MUL | CURRENT_TIMESTAMP |                |

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

4 rows in set (0.00 sec)