Apex domains cannot have CNAME records.
Question and response
Question
dig iol.cz. NS
Answer
iol.cz. 5 NS ns2.tel.cz.
iol.cz. 5 NS dns.iol.cz.
Authority
The response has no authority records.
Additional
dns.iol.cz. 5 A 194.228.2.61
ns2.tel.cz. 5 A 194.228.2.1
dns.iol.cz. 5 AAAA 2a00:1028:100:102:0:0:0:61
ns2.tel.cz. 5 AAAA 2a00:1028:1:205:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0