Apex domains cannot have CNAME records.
Question and response
Question
dig karlov.mff.cuni.cz. NS
Answer
karlov.mff.cuni.cz. 5 NS golias.ruk.cuni.cz.
karlov.mff.cuni.cz. 5 NS jay.karlov.mff.cuni.cz.
karlov.mff.cuni.cz. 5 NS owl.karlov.mff.cuni.cz.
karlov.mff.cuni.cz. 5 NS david.ruk.cuni.cz.
Authority
The response has no authority records.
Additional
david.ruk.cuni.cz. 5 A 78.128.213.242
golias.ruk.cuni.cz. 5 A 195.113.0.2
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