Unexpected error: Incorrect ASSOCIATIVE ARRAY KEY value

SET sql_mode=ORACLE;

DELIMITER $$

DECLARE

  TYPE t IS TABLE OF INT INDEX BY BIGINT;

  a t;

BEGIN

  a(-9223372036854775807-1):= 10;   -- key is LONGLONG_MIN

  SELECT a(-9223372036854775807-1) AS c1;

END;

$$

DELIMITER ;

ERROR 1525 (HY000): Incorrect ASSOCIATIVE ARRAY KEY value: '-9223372036854775808'