Apex domains cannot have CNAME records.
Question and response
Question
dig ubivox.dk. NS
Answer
ubivox.dk. 5 NS ns4.ubivox.org.
ubivox.dk. 5 NS ns1.ubivox.org.
ubivox.dk. 5 NS ns2.ubivox.org.
ubivox.dk. 5 NS ns3.ubivox.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0