Apex domains cannot have CNAME records.
Question and response
Question
dig poradca.sk. NS
Answer
poradca.sk. 5 NS ns1.dnsbackup.net.
poradca.sk. 5 NS ns2.dnsbackup.net.
poradca.sk. 5 NS ns1.nameserver.sk.
poradca.sk. 5 NS ns2.nameserver.sk.
Authority
The response has no authority records.
Additional
ns1.dnsbackup.net. 5 A 178.62.60.201
ns1.nameserver.sk. 5 A 185.65.223.35
ns2.dnsbackup.net. 5 A 185.65.223.35
ns2.nameserver.sk. 5 A 217.31.48.201
ns1.dnsbackup.net. 5 AAAA 2a03:b0c0:1:d0:0:0:1090:2001
ns1.nameserver.sk. 5 AAAA 2a00:1e40:a:53:0:0:0:5
ns2.dnsbackup.net. 5 AAAA 2a00:1e40:a:53:0:0:0:5
ns2.nameserver.sk. 5 AAAA 2001:1ab0:7e1e:2:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0