Apex domains cannot have CNAME records.
Question and response
Question
dig dns.cv. NS
Answer
dns.cv. 5 NS ns.dns.cv.
dns.cv. 5 NS ns1.dns.cv.
dns.cv. 5 NS cv01.dns.pt.
dns.cv. 5 NS dns-ext01.dns.pt.
Authority
The response has no authority records.
Additional
ns.dns.cv. 5 A 41.221.192.220
ns1.dns.cv. 5 A 41.221.192.219
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0