Apex domains cannot have CNAME records.
Question and response
Question
dig zcom.cz. NS
Answer
zcom.cz. 5 NS ns1.zcom.cz.
zcom.cz. 5 NS ns2.zcom.cz.
Authority
The response has no authority records.
Additional
ns1.zcom.cz. 5 A 185.178.172.22
ns2.zcom.cz. 5 A 89.185.230.230
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0