Apex domains cannot have CNAME records.
Question and response
Question
dig setr7p.cz. NS
Answer
setr7p.cz. 5 NS ns-757.awsdns-30.net.
setr7p.cz. 5 NS ns-1389.awsdns-45.org.
setr7p.cz. 5 NS ns-2019.awsdns-60.co.uk.
setr7p.cz. 5 NS ns-110.awsdns-13.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0