Apex domains cannot have CNAME records.
Question and response
Question
dig smarth5h6.cz. NS
Answer
smarth5h6.cz. 5 NS ns-1316.awsdns-36.org.
smarth5h6.cz. 5 NS ns-1671.awsdns-16.co.uk.
smarth5h6.cz. 5 NS ns-469.awsdns-58.com.
smarth5h6.cz. 5 NS ns-600.awsdns-11.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0