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