Apex domains cannot have CNAME records.
Question and response
Question
dig edera.cz. NS
Answer
edera.cz. 5 NS secondary.edera.cz.
edera.cz. 5 NS ns.edera.cz.
Authority
The response has no authority records.
Additional
ns.edera.cz. 5 A 77.236.192.142
secondary.edera.cz. 5 A 37.157.197.66
ns.edera.cz. 5 AAAA 2a03:f280:11:1:0:0:0:2
secondary.edera.cz. 5 AAAA 2a02:2b88:6:2dd6:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0