Apex domains cannot have CNAME records.
Question and response
Question
dig decks.de. NS
Answer
decks.de. 5 NS robotns2.second-ns.de.
decks.de. 5 NS ns1.first-ns.de.
decks.de. 5 NS robotns3.second-ns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0