Apex domains cannot have CNAME records.
Question and response
Question
dig secure-dns.eu. NS
Answer
secure-dns.eu. 5 NS ns1.eurodns.com.
secure-dns.eu. 5 NS ns3.eurodns.com.
secure-dns.eu. 5 NS ns2.eurodns.com.
secure-dns.eu. 5 NS ns4.eurodns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0