Apex domains cannot have CNAME records.
Question and response
Question
dig xnameserver.de. NS
Answer
xnameserver.de. 5 NS c.xnameserver.de.
xnameserver.de. 5 NS a.xnameserver.de.
xnameserver.de. 5 NS b.xnameserver.de.
Authority
The response has no authority records.
Additional
a.xnameserver.de. 5 A 192.174.68.104
b.xnameserver.de. 5 A 176.97.158.104
c.xnameserver.de. 5 A 45.87.158.53
a.xnameserver.de. 5 AAAA 2001:67c:1bc:0:0:0:0:104
b.xnameserver.de. 5 AAAA 2001:67c:10b8:0:0:0:0:104
c.xnameserver.de. 5 AAAA 2a02:d500:0:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0