Apex domains cannot have CNAME records.
Question and response
Question
dig ropos.de. NS
Answer
ropos.de. 5 NS ns1112.ui-dns.de.
ropos.de. 5 NS ns1122.ui-dns.org.
ropos.de. 5 NS ns1028.ui-dns.com.
ropos.de. 5 NS ns1116.ui-dns.biz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0