JSON_MERGE accepts arguments in invalid format

MySQL [test]> SELECT JSON_MERGE('a','b');

ERROR 3141 (22032): Invalid JSON text in argument 1 to function json_merge: "Invalid value." at position 0.

MariaDB [test]> SELECT JSON_MERGE('a','b');

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

| JSON_MERGE('a','b') |

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

| ["a", "b"]          |

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

1 row in set (0.00 sec)