Apex domains cannot have CNAME records.
Question and response
Question
dig hdc2.cz. NS
Answer
hdc2.cz. 5 NS ns.hdc1.cz.
hdc2.cz. 5 NS ns.hdc2.cz.
Authority
The response has no authority records.
Additional
ns.hdc1.cz. 5 A 185.68.70.133
ns.hdc2.cz. 5 A 185.68.70.134
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0