Apex domains cannot have CNAME records.
Question and response
Question
dig stable.qloud-b.yandex.net. NS
Answer
stable.qloud-b.yandex.net. 5 NS ns6.yandex.ru.
stable.qloud-b.yandex.net. 5 NS ns5.yandex.ru.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0