Apex domains cannot have CNAME records.
Question and response
Question
dig cbsn.cz. NS
Answer
cbsn.cz. 5 NS ns1.cbsn.cz.
cbsn.cz. 5 NS ns3.cbsn.cz.
cbsn.cz. 5 NS ns2.cbsn.cz.
Authority
The response has no authority records.
Additional
ns1.cbsn.cz. 5 A 81.25.16.219
ns2.cbsn.cz. 5 A 81.25.27.10
ns3.cbsn.cz. 5 A 37.157.199.146
ns1.cbsn.cz. 5 AAAA 2a00:1800:0:ffff:0:0:0:1
ns2.cbsn.cz. 5 AAAA 2a00:1800:ff0:0:0:0:0:10
ns3.cbsn.cz. 5 AAAA 2a02:2b88:2:1:0:0:15a7:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0