Apex domains cannot have CNAME records.
Question and response
Question
dig start-dns.de. NS
Answer
start-dns.de. 4 NS ns3.start-dns.de.
start-dns.de. 4 NS ns1.start-dns.de.
start-dns.de. 4 NS ns2.start-dns.de.
Authority
The response has no authority records.
Additional
ns1.start-dns.de. 4 A 87.238.196.5
ns2.start-dns.de. 4 A 178.254.5.5
ns3.start-dns.de. 4 A 178.254.63.7
ns1.start-dns.de. 4 AAAA 2a00:6800:200:1:0:0:0:5
ns2.start-dns.de. 4 AAAA 2a00:6800:200:2:0:0:0:5
ns3.start-dns.de. 4 AAAA 2a02:2800:250:0:0:0:0:7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0