Apex domains cannot have CNAME records.
Question and response
Question
dig cortex.cz. NS
Answer
cortex.cz. 5 NS ns-21.cortex.cz.
cortex.cz. 5 NS ns-11.cortex.cz.
Authority
The response has no authority records.
Additional
ns-11.cortex.cz. 5 A 185.250.102.194
ns-21.cortex.cz. 5 A 185.250.102.10
ns-11.cortex.cz. 5 AAAA 2a0e:e581:1:aaaa:0:0:0:53
ns-21.cortex.cz. 5 AAAA 2a0e:e581:2:aaaa:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0