Apex domains cannot have CNAME records.
Question and response
Question
dig bartanet.cz. NS
Answer
bartanet.cz. 5 NS ns.bartanet.cz.
bartanet.cz. 5 NS ns3.bartanet.cz.
bartanet.cz. 5 NS ns2.bartanet.cz.
Authority
The response has no authority records.
Additional
ns.bartanet.cz. 5 A 192.162.96.10
ns2.bartanet.cz. 5 A 192.162.97.1
ns3.bartanet.cz. 5 A 192.162.96.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0