Apex domains cannot have CNAME records.
Question and response
Question
dig smarts7d6.cz. NS
Answer
smarts7d6.cz. 5 NS ns-137.awsdns-17.com.
smarts7d6.cz. 5 NS ns-1866.awsdns-41.co.uk.
smarts7d6.cz. 5 NS ns-1531.awsdns-63.org.
smarts7d6.cz. 5 NS ns-585.awsdns-09.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0