JSON_KEYS accepts arguments in invalid format

MariaDB [test]> SELECT JSON_KEYS('foo');

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

| JSON_KEYS('foo') |

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

| NULL             |

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

1 row in set (0.00 sec)

MySQL [test]> SELECT JSON_KEYS('foo');

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