Apex domains cannot have CNAME records.
Question and response
Question
dig sovereignit.de. NS
Answer
sovereignit.de. 5 NS ns1107.ui-dns.de.
sovereignit.de. 5 NS ns1092.ui-dns.org.
sovereignit.de. 5 NS ns1069.ui-dns.biz.
sovereignit.de. 5 NS ns1120.ui-dns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0