Apex domains cannot have CNAME records.
Question and response
Question
dig ui-dns.de. NS
Answer
ui-dns.de. 5 NS ns-de.ui-dns.com.
ui-dns.de. 5 NS ns-de.ui-dns.org.
ui-dns.de. 5 NS ns-de.ui-dns.de.
ui-dns.de. 5 NS ns-de.ui-dns.biz.
Authority
The response has no authority records.
Additional
ns-de.ui-dns.de. 5 A 185.132.32.193
ns-de.ui-dns.de. 5 A 217.160.80.193
ns-de.ui-dns.de. 5 AAAA 2001:8d8:fe:53:0:d9a0:50c1:100
ns-de.ui-dns.de. 5 AAAA 2607:f1c0:fe:53:185:132:32:193
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0