Apex domains cannot have CNAME records.
Question and response
Question
dig purteldns.de. NS
Answer
purteldns.de. 4 NS ns2.purteldns.de.
purteldns.de. 4 NS ns3.purteldns.de.
purteldns.de. 4 NS ns1.purteldns.de.
Authority
The response has no authority records.
Additional
ns1.purteldns.de. 4 A 185.39.85.4
ns2.purteldns.de. 4 A 185.39.86.4
ns3.purteldns.de. 4 A 213.133.97.135
ns1.purteldns.de. 4 AAAA 2a01:41e0:110:0:0:0:0:4
ns2.purteldns.de. 4 AAAA 2a01:41e0:10:0:0:0:0:4
ns3.purteldns.de. 4 AAAA 2a01:4f8:130:9181:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0