Apex domains cannot have CNAME records.
Question and response
Question
dig ruk.cuni.cz. NS
Answer
ruk.cuni.cz. 5 NS ruzenka.prf.cuni.cz.
ruk.cuni.cz. 5 NS david.ruk.cuni.cz.
ruk.cuni.cz. 5 NS cuce.ruk.cuni.cz.
ruk.cuni.cz. 5 NS golias.ruk.cuni.cz.
Authority
The response has no authority records.
Additional
cuce.ruk.cuni.cz. 5 A 195.113.0.8
david.ruk.cuni.cz. 5 A 78.128.213.242
golias.ruk.cuni.cz. 5 A 195.113.0.2
cuce.ruk.cuni.cz. 5 AAAA 2001:718:1e03:1:0:0:0:8
david.ruk.cuni.cz. 5 AAAA 2001:718:1203:502:0:0:0:2
golias.ruk.cuni.cz. 5 AAAA 2001:718:1e03:1:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0