Apex domains cannot have CNAME records.
Question and response
Question
dig dnsservers.dk. NS
Answer
dnsservers.dk. 5 NS ns1.dnsservers.dk.
dnsservers.dk. 5 NS ns3.dnsservers.dk.
dnsservers.dk. 5 NS ns2.dnsservers.dk.
Authority
The response has no authority records.
Additional
ns1.dnsservers.dk. 5 A 217.30.38.181
ns2.dnsservers.dk. 5 A 217.30.38.27
ns3.dnsservers.dk. 5 A 217.30.38.30
ns1.dnsservers.dk. 5 AAAA 2001:1448:0:a800:0:0:0:53
ns2.dnsservers.dk. 5 AAAA 2001:1448:0:a500:0:0:0:53
ns3.dnsservers.dk. 5 AAAA 2001:1448:0:a600:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0