Apex domains cannot have CNAME records.
Question and response
Question
dig n.mff.cuni.cz. NS
Answer
n.mff.cuni.cz. 2 NS ns.ms.mff.cuni.cz.
n.mff.cuni.cz. 2 NS sns.ms.mff.cuni.cz.
n.mff.cuni.cz. 2 NS ns.n.mff.cuni.cz.
Authority
The response has no authority records.
Additional
ns.n.mff.cuni.cz. 2 A 195.113.16.2
ns.ms.mff.cuni.cz. 2 A 195.113.20.71
sns.ms.mff.cuni.cz. 2 A 195.113.20.77
ns.n.mff.cuni.cz. 2 AAAA 2001:718:1e03:e01:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0