Apex domains cannot have CNAME records.
Question and response
Question
dig nwlink.spb.ru. NS
Answer
nwlink.spb.ru. 5 NS ns.pulkov.spb.ru.
nwlink.spb.ru. 5 NS ns.nalitch.spb.ru.
Authority
The response has no authority records.
Additional
ns.pulkov.spb.ru. 5 A 178.252.64.1
ns.nalitch.spb.ru. 5 A 178.252.127.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0