Apex domains cannot have CNAME records.
Question and response
Question
dig rionet.cz. NS
Answer
rionet.cz. 5 NS ns1.riomedia.cz.
rionet.cz. 5 NS ns2.riomedia.cz.
Authority
The response has no authority records.
Additional
ns1.riomedia.cz. 5 A 94.74.192.250
ns2.riomedia.cz. 5 A 94.74.192.245
ns1.riomedia.cz. 5 AAAA 2a02:350:0:101:0:0:0:3
ns2.riomedia.cz. 5 AAAA 2a02:350:0:202:0:0:0:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0