Apex domains cannot have CNAME records.
Question and response
Question
dig gtsi.sk. NS
Answer
gtsi.sk. 5 NS ns.nextra.sk.
gtsi.sk. 5 NS ns1.nextra.sk.
gtsi.sk. 5 NS dns.gtsi.sk.
Authority
The response has no authority records.
Additional
ns.nextra.sk. 5 A 195.168.1.53
ns1.nextra.sk. 5 A 195.168.1.52
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0