Apex domains cannot have CNAME records.
Question and response
Question
dig SIBSVST.ru. NS
Answer
SIBSVST.ru. 5 NS ns2.ip-stream.ru.
SIBSVST.ru. 5 NS ns2.SIBSVST.ru.
SIBSVST.ru. 5 NS ns1.SIBSVST.ru.
SIBSVST.ru. 5 NS ns1.ip-stream.ru.
Authority
The response has no authority records.
Additional
ns1.ip-stream.ru. 5 A 91.202.68.30
ns2.ip-stream.RU. 5 A 91.202.68.28
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0