Apex domains cannot have CNAME records.
Question and response
Question
dig vlt81.de. NS
Answer
vlt81.de. 5 NS ns1.1984.is.
vlt81.de. 5 NS dns.vlt81.de.
vlt81.de. 5 NS ns0.1984.is.
vlt81.de. 5 NS ns2.1984.is.
Authority
The response has no authority records.
Additional
dns.vlt81.de. 5 A 176.9.242.147
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0