Apex domains cannot have CNAME records.
Question and response
Question
dig zsttk.ru. NS
Answer
zsttk.ru. 5 NS ns.zsttk.ru.
zsttk.ru. 5 NS ns1.transtelecom.net.
zsttk.ru. 5 NS ns.transtelecom.net.
Authority
The response has no authority records.
Additional
ns.zsttk.ru. 5 A 80.89.128.8
ns.transtelecom.net. 5 A 188.43.29.3
ns1.transtelecom.net. 5 A 188.43.30.219
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0