Apex domains cannot have CNAME records.
Question and response
Question
dig se6ruq.cz. NS
Answer
se6ruq.cz. 5 NS ns-1936.awsdns-50.co.uk.
se6ruq.cz. 5 NS ns-805.awsdns-36.net.
se6ruq.cz. 5 NS ns-46.awsdns-05.com.
se6ruq.cz. 5 NS ns-1266.awsdns-30.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0