Apex domains cannot have CNAME records.
Question and response
Question
dig etn.cz. NS
Answer
etn.cz. 5 NS ns3.etnetera.eu.
etn.cz. 5 NS ns1.etn.cz.
Authority
The response has no authority records.
Additional
ns1.etn.cz. 5 A 178.238.34.21
ns3.etnetera.eu. 5 A 4.182.218.254
ns1.etn.cz. 5 AAAA 2a01:430:3c:1:34:0:0:21
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0