Apex domains cannot have CNAME records.
Question and response
Question
dig tuzvo.sk. NS
Answer
tuzvo.sk. 5 NS dnsa.tuzvo.sk.
tuzvo.sk. 5 NS uvt.tuzvo.sk.
tuzvo.sk. 5 NS sun.uakom.sk.
tuzvo.sk. 5 NS dnsb.tuzvo.sk.
Authority
The response has no authority records.
Additional
sun.uakom.sk. 5 A 192.108.131.11
uvt.tuzvo.sk. 5 A 194.160.170.5
dnsb.tuzvo.sk. 5 A 194.160.170.6
sun.uakom.sk. 5 AAAA 2001:4118:100:1:0:0:0:11
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0