Apex domains cannot have CNAME records.
Question and response
Question
dig united-internet.org. NS
Answer
united-internet.org. 5 NS ns-ui.ui-dns.biz.
united-internet.org. 5 NS ns-ui.ui-dns.com.
united-internet.org. 5 NS ns-ui.ui-dns.org.
united-internet.org. 5 NS ns-ui.ui-dns.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0