Apex domains cannot have CNAME records.
Question and response
Question
dig wn-dns.de. NS
Answer
wn-dns.de. 5 NS dns.wn-dns.de.
wn-dns.de. 5 NS ns1.waaf.net.
wn-dns.de. 5 NS ns3.waaf.net.
Authority
The response has no authority records.
Additional
dns.wn-dns.de. 5 A 46.101.245.225
dns.wn-dns.de. 5 AAAA 2a03:b0c0:3:d0:0:0:28:b001
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0