Apex domains cannot have CNAME records.
Question and response
Question
dig atcomp.cz. NS
Answer
atcomp.cz. 5 NS ns3.ha-vel.com.
atcomp.cz. 5 NS ns2.ha-vel.cz.
atcomp.cz. 5 NS ns6.atcomp.cz.
Authority
The response has no authority records.
Additional
ns2.ha-vel.cz. 5 A 81.30.231.20
ns3.ha-vel.com. 5 A 46.105.29.243
ns6.atcomp.cz. 5 A 193.85.25.23
ns2.ha-vel.cz. 5 AAAA 2a02:570:20:0:0:0:0:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0