Apex domains cannot have CNAME records.
Question and response
Question
dig sippeer.dk. NS
Answer
sippeer.dk. 5 NS eric.ns.cloudflare.com.
sippeer.dk. 5 NS ruth.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0