Apex domains cannot have CNAME records.
Question and response
Question
dig utia.cas.cz. NS
Answer
utia.cas.cz. 5 NS academy.cas.cz.
utia.cas.cz. 5 NS nsa.ces.net.
utia.cas.cz. 5 NS visla.utia.cas.cz.
Authority
The response has no authority records.
Additional
nsa.ces.net. 4 A 195.113.144.205
visla.utia.cas.cz. 4 A 147.231.12.1
academy.cas.cz. 4 A 147.231.58.1
nsa.ces.net. 4 AAAA 2001:718:1:1:0:0:144:205
visla.utia.cas.cz. 4 AAAA 2001:718:401:1:0:0:0:1
academy.cas.cz. 4 AAAA 2001:718:404:1002:147:231:58:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0