Apex domains cannot have CNAME records.
Question and response
Question
dig rzone.de. NS
Answer
rzone.de. 5 NS ns2.rzone.de.
rzone.de. 5 NS ns1.rzone.de.
rzone.de. 5 NS ns-anyslv.ui-dns.de.
rzone.de. 5 NS ns4.rzone.de.
Authority
The response has no authority records.
Additional
ns1.rzone.de. 5 A 193.141.3.6
ns2.rzone.de. 5 A 81.169.144.234
ns4.rzone.de. 5 A 192.166.192.224
ns4.rzone.de. 5 AAAA 2a01:238:e100:f003:0:0:0:110
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0