Apex domains cannot have CNAME records.
Question and response
Question
dig iol.sk. NS
Answer
iol.sk. 5 NS ns1.iol.sk.
iol.sk. 5 NS ns.iol.sk.
Authority
The response has no authority records.
Additional
ns.iol.sk. 5 A 193.58.193.5
ns1.iol.sk. 5 A 195.12.151.2
ns1.iol.sk. 5 A 195.12.151.3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0