Apex domains cannot have CNAME records.
Question and response
Question
dig bitternet.ua. NS
Answer
bitternet.ua. 5 NS ns3.bitternet.ua.
bitternet.ua. 5 NS ns2.bitternet.ua.
bitternet.ua. 5 NS ns1.bitternet.ua.
Authority
The response has no authority records.
Additional
ns1.bitternet.ua. 5 A 185.53.79.2
ns2.bitternet.ua. 5 A 185.53.79.3
ns3.bitternet.ua. 5 A 178.165.81.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0