Apex domains cannot have CNAME records.
Question and response
Question
dig klausen.dk. NS
Answer
klausen.dk. 5 NS ns1.first-ns.de.
klausen.dk. 5 NS ns1.klausen.dk.
klausen.dk. 5 NS ns2.he.net.
Authority
The response has no authority records.
Additional
ns1.klausen.dk. 5 A 157.90.24.29
ns2.he.net. 5 A 216.218.131.2
ns1.klausen.dk. 5 AAAA 2a01:4f8:c2c:8952:0:0:0:0
ns2.he.net. 5 AAAA 2001:470:200:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0