Apex domains cannot have CNAME records.
Question and response
Question
dig netdatacomm.cz. NS
Answer
netdatacomm.cz. 5 NS ns2.netdatacomm.cz.
netdatacomm.cz. 5 NS ns.netdatacomm.cz.
Authority
The response has no authority records.
Additional
ns.netdatacomm.cz. 5 A 176.74.128.10
ns2.netdatacomm.cz. 5 A 176.74.128.11
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0