Apex domains cannot have CNAME records.
Question and response
Question
dig seh6tr.cz. NS
Answer
seh6tr.cz. 5 NS ns-1616.awsdns-10.co.uk.
seh6tr.cz. 5 NS ns-115.awsdns-14.com.
seh6tr.cz. 5 NS ns-959.awsdns-55.net.
seh6tr.cz. 5 NS ns-1129.awsdns-13.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0