Apex domains cannot have CNAME records.
Question and response
Question
dig cuni.cz. NS
Answer
cuni.cz. 4 NS david.ruk.cuni.cz.
cuni.cz. 4 NS golias.ruk.cuni.cz.
cuni.cz. 4 NS nsa.ces.net.
cuni.cz. 4 NS cuce.ruk.cuni.cz.
Authority
The response has no authority records.
Additional
nsa.ces.net. 4 A 195.113.144.205
cuce.ruk.cuni.cz. 4 A 195.113.0.8
david.ruk.cuni.cz. 4 A 78.128.213.242
golias.ruk.cuni.cz. 4 A 195.113.0.2
nsa.ces.net. 4 AAAA 2001:718:1:1:0:0:144:205
cuce.ruk.cuni.cz. 4 AAAA 2001:718:1e03:1:0:0:0:8
david.ruk.cuni.cz. 4 AAAA 2001:718:1203:502:0:0:0:2
golias.ruk.cuni.cz. 4 AAAA 2001:718:1e03:1:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0