Apex domains cannot have CNAME records.
Question and response
Question
dig v-dns.de. NS
Answer
v-dns.de. 5 NS ns02.v-dns.de.
v-dns.de. 5 NS ns01.v-dns.de.
Authority
The response has no authority records.
Additional
ns01.v-dns.de. 5 A 192.174.68.13
ns02.v-dns.de. 5 A 176.97.158.13
ns01.v-dns.de. 5 AAAA 2001:67c:1bc:0:0:0:0:13
ns02.v-dns.de. 5 AAAA 2001:67c:10b8:0:0:0:0:13
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0