Apex domains cannot have CNAME records.
Question and response
Question
dig nej.cz. NS
Answer
nej.cz. 5 NS ns.nej.cz.
nej.cz. 5 NS sns.nej.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0