Apex domains cannot have CNAME records.
Question and response
Question
dig cez.cz. NS
Answer
cez.cz. 5 NS ns10.cez.cz.
cez.cz. 5 NS ns20.cez.cz.
Authority
The response has no authority records.
Additional
ns10.cez.cz. 5 A 89.111.73.200
ns20.cez.cz. 5 A 89.111.77.200
ns10.cez.cz. 5 AAAA 2a01:a040:2004:8100:0:0:0:1
ns20.cez.cz. 5 AAAA 2a01:a040:2004:100:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0