Apex domains cannot have CNAME records.
Question and response
Question
dig yellducal.com. NS
Answer
yellducal.com. 4 NS ns1030.ui-dns.de.
yellducal.com. 4 NS ns1019.ui-dns.com.
yellducal.com. 4 NS ns1120.ui-dns.org.
yellducal.com. 4 NS ns1085.ui-dns.biz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0