Thank you for the comment!
I’ve checked the behavior and previous version does not indeed cover the case with box.NULL. Suitable fix and tests were added. 
  
>Четверг, 7 ноября 2019, 17:08 +03:00 от Бабин Олег <olegrok@tarantool.org>:
> 
>Maria, thanks for your patch!
>
>But as I see you don't consider the case of box.NULL that has 'cdata' type, but doesn't have
>__serialize metamethod.
>
>It will lead to an error:
>```
>- error: '[string "return x:__serialize()"]:1: ''void *'' has no member named ''__serialize'''
>...
>```
>>Четверг, 7 ноября 2019, 16:47 +03:00 от Maria < maria.khaydich@tarantool.org >:
>>
>>+ if type(res) == 'cdata' then
>>+ results[n] = res:__serialize()
>>+ end
>
>-- 
>Oleg Babin
 
 
--
Maria Khaydich