JSON_QUOTE() - return NULL on numbers

I'm not sure if this is really a bug, but why JSON_QUOTE () returns NULL when the input parameter is not a string, but a number?

My opinion in that case, is that the return value must be the same as input number.

MariaDB [test]> SELECT JSON_QUOTE(90211);

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

| JSON_QUOTE(90211) |

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

| NULL              |

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

1 row in set (0.00 sec)