Apex domains cannot have CNAME records.
Question and response
Question
dig supersrv.de. NS
Answer
supersrv.de. 5 NS third-dns.netcup.net.
supersrv.de. 5 NS second-dns.netcup.net.
supersrv.de. 5 NS root-dns.netcup.net.
Authority
The response has no authority records.
Additional
root-dns.netcup.net. 3 A 46.38.225.225
third-dns.netcup.net. 3 A 188.68.63.68
second-dns.netcup.net. 3 A 37.221.199.199
root-dns.netcup.net. 3 AAAA 2a03:4000:0:1:0:0:0:e1e1
third-dns.netcup.net. 3 AAAA 2a03:4001:0:106:0:0:0:3f44
second-dns.netcup.net. 3 AAAA 2a03:4000:2:24b:0:0:0:c7c7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0