Apex domains cannot have CNAME records.
Question and response
Question
dig heysender.dk. NS
Answer
heysender.dk. 5 NS ns2.curanet.dk.
heysender.dk. 5 NS ns1.curanet.dk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0