Apex domains cannot have CNAME records.
Question and response
Question
dig ui-dns.org. NS
Answer
ui-dns.org. 5 NS ns-org.ui-dns.de.
ui-dns.org. 5 NS ns-org.ui-dns.org.
ui-dns.org. 5 NS ns-org.ui-dns.com.
ui-dns.org. 5 NS ns-org.ui-dns.biz.
Authority
The response has no authority records.
Additional
ns-org.ui-dns.org. 5 A 185.132.34.194
ns-org.ui-dns.org. 5 A 217.160.82.194
ns-org.ui-dns.org. 5 AAAA 2001:8d8:fe:53:0:d9a0:52c2:100
ns-org.ui-dns.org. 5 AAAA 2607:f1c0:fe:53:185:132:34:194
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0