Apex domains cannot have CNAME records.
Question and response
Question
dig dk.customer.tdc.net. NS
Answer
dk.customer.tdc.net. 5 NS auth06.ns.tdc.net.
dk.customer.tdc.net. 5 NS auth05.ns.tdc.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0