Apex domains cannot have CNAME records.
Question and response
Question
dig vnet.sk. NS
Answer
vnet.sk. 5 NS ns.vnet.cz.
vnet.sk. 5 NS ns.vnet.eu.
vnet.sk. 5 NS ns.vnet.sk.
vnet.sk. 5 NS outsider.vnet.sk.
Authority
The response has no authority records.
Additional
ns.vnet.cz. 5 A 217.73.16.2
ns.vnet.eu. 5 A 86.110.224.30
ns.vnet.sk. 5 A 217.73.17.2
outsider.vnet.sk. 5 A 217.119.115.200
ns.vnet.sk. 5 AAAA 2a01:390:1:0:0:0:0:cafe
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0