Apex domains cannot have CNAME records.
Question and response
Question
dig dnsever.com. NS
Answer
dnsever.com. 5 NS ns7.dnsever.co.kr.
dnsever.com. 5 NS ns7.dnsever.org.
dnsever.com. 5 NS ns7.dnsever.co.uk.
dnsever.com. 5 NS ns7.dnsever.com.
dnsever.com. 5 NS ns7.dnsever.net.
Authority
The response has no authority records.
Additional
ns7.dnsever.com. 5 A 51.77.113.149
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0