Apex domains cannot have CNAME records.
Question and response
Question
dig ri-server.de. NS
Answer
ri-server.de. 5 NS b.ns14.net.
ri-server.de. 5 NS a.ns14.net.
ri-server.de. 5 NS d.ns14.net.
ri-server.de. 5 NS c.ns14.net.
Authority
The response has no authority records.
Additional
a.ns14.net. 5 A 217.160.80.160
b.ns14.net. 5 A 217.160.81.160
c.ns14.net. 5 A 185.132.35.160
d.ns14.net. 5 A 185.132.34.160
a.ns14.net. 5 AAAA 2001:8d8:fe:53:217:160:80:160
b.ns14.net. 5 AAAA 2001:8d8:fe:53:217:160:81:160
c.ns14.net. 5 AAAA 2607:f1c0:fe:53:185:132:35:160
d.ns14.net. 5 AAAA 2607:f1c0:fe:53:185:132:34:160
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0