Apex domains cannot have CNAME records.
Question and response
Question
dig anet.dk. NS
Answer
anet.dk. 5 NS ns02.athena-hosting.dk.
anet.dk. 5 NS ns01.athena-hosting.dk.
anet.dk. 5 NS ns03.athena-hosting.dk.
Authority
The response has no authority records.
Additional
ns01.athena-hosting.dk. 5 A 217.30.38.181
ns02.athena-hosting.dk. 5 A 217.30.38.27
ns03.athena-hosting.dk. 5 A 217.30.38.30
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0