Apex domains cannot have CNAME records.
Question and response
Question
dig samek.cz. NS
Answer
samek.cz. 5 NS www.benesovi.eu.
samek.cz. 5 NS tiberius.samek.cz.
Authority
The response has no authority records.
Additional
www.benesovi.eu. 5 A 93.190.50.11
tiberius.samek.cz. 5 A 185.33.137.237
www.benesovi.eu. 5 AAAA 2a02:c600:1:1337:0:0:0:1
tiberius.samek.cz. 5 AAAA 2a01:6400:ff00:0:0:0:0:ed
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0