Apex domains cannot have CNAME records.
Question and response
Question
dig smartu5s6.cz. NS
Answer
smartu5s6.cz. 5 NS ns-1189.awsdns-20.org.
smartu5s6.cz. 5 NS ns-1617.awsdns-10.co.uk.
smartu5s6.cz. 5 NS ns-437.awsdns-54.com.
smartu5s6.cz. 5 NS ns-789.awsdns-34.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0