Apex domains cannot have CNAME records.
Question and response
Question
dig gymfed.cz. NS
Answer
gymfed.cz. 4 NS bt.satoya.cz.
gymfed.cz. 4 NS ns.satoya.cz.
Authority
The response has no authority records.
Additional
bt.satoya.cz. 4 A 212.24.129.94
ns.satoya.cz. 4 A 94.142.233.142
bt.satoya.cz. 4 AAAA 2001:4de8:b0ba:ff26:0:0:0:94
ns.satoya.cz. 4 AAAA 2a03:a900:0:303:0:0:0:142
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0