Apex domains cannot have CNAME records.
Question and response
Question
dig nc.de. NS
Answer
nc.de. 4 NS ns3.netcologne.de.
nc.de. 4 NS ns1.netcologne.de.
Authority
The response has no authority records.
Additional
ns1.netcologne.de. 4 A 194.8.194.70
ns3.netcologne.de. 4 A 194.8.197.10
ns1.netcologne.de. 4 AAAA 2001:4dd0:100:1020:53:1:0:1
ns3.netcologne.de. 4 AAAA 2001:4dd0:100:4220:53:1:0:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0