NAME_CONST with NULL argument produces unexpected error message

MySQL manual says that NAME_CONST is for internal use only; MariaDB's doesn't. However, both MySQL and MariaDB allow it to be used with "normal" arguments:

MariaDB [test]> select NAME_CONST('foo','bar');

+------+

| foo  |

+------+

| bar  |

+------+

1 row in set (0.00 sec)

but both suddenly return "for internal use error" when the first argument is NULL:

MariaDB [test]> select NAME_CONST(NULL,'bar');

ERROR 1382 (HY000): The 'NAME_CONST' syntax is reserved for purposes internal to the MariaDB server

It appears to be inconsistent, I would expect the error about wrong first argument (as it's supposed to be a valid identifier and naturally shouldn't be NULL).