Apex domains cannot have CNAME records.
Question and response
Question
dig sewra5.cz. NS
Answer
sewra5.cz. 5 NS ns-1670.awsdns-16.co.uk.
sewra5.cz. 5 NS ns-539.awsdns-03.net.
sewra5.cz. 5 NS ns-395.awsdns-49.com.
sewra5.cz. 5 NS ns-1384.awsdns-45.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0