ROW_NUMBER in SIGNAL/RESIGNAL causes a syntax error

DROP TABLE t1;

delimiter $

BEGIN NOT ATOMIC

  DECLARE EXIT HANDLER FOR SQLEXCEPTION

    SIGNAL SQLSTATE '45000' SET ROW_NUMBER= 5;

  DROP TABLE t1;

END $

delimiter ;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'ROW_NUMBER= 5;

  DROP TABLE t1;

MariaDB [test]> delimiter ;

MariaDB [test]> DROP TABLE t1;

ERROR 1051 (42S02): Unknown table 'test.t1'

MariaDB [test]> delimiter $

MariaDB [test]> BEGIN NOT ATOMIC

    ->   DECLARE EXIT HANDLER FOR SQLEXCEPTION

    ->     SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT= 'my signal here';

    ->   DROP TABLE t1;

    -> END $

ERROR 1644 (45000): my signal here

MariaDB [test]> delimiter ;