Apex domains cannot have CNAME records.
Question and response
Question
dig mioweb.cz. NS
Answer
mioweb.cz. 5 NS ns-1103.awsdns-09.org.
mioweb.cz. 5 NS ns-369.awsdns-46.com.
mioweb.cz. 5 NS ns-806.awsdns-36.net.
mioweb.cz. 5 NS ns-1723.awsdns-23.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0