Apex domains cannot have CNAME records.
Question and response
Question
dig default.cz. NS
Answer
default.cz. 5 NS ns.default.cz.
default.cz. 5 NS ns2.default.cz.
Authority
The response has no authority records.
Additional
ns.default.cz. 5 A 82.208.6.1
ns2.default.cz. 5 A 93.185.101.31
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0