weird behavior with json extract

select json_extract('', '$');

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

| json_extract('', '$') |

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

| NULL                  |

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

1 row in set, 1 warning (0.000 sec)

insert into tab(col) values(NULL);

Query OK, 1 row affected (0.002 sec)

insert into tab(col) values(json_extract('', '$'));

ERROR 4037 (HY000): Unexpected end of JSON text in argument 1 to function 'json_extract'