Apex domains cannot have CNAME records.
Question and response
Question
dig ten.cz. NS
Answer
ten.cz. 3 NS decsys.vsb.cz.
ten.cz. 3 NS nsa.ten.cz.
ten.cz. 3 NS nsa.ces.net.
ten.cz. 3 NS nsa.cesnet.cz.
Authority
The response has no authority records.
Additional
nsa.ces.net. 3 A 195.113.144.205
nsa.ten.cz. 3 A 81.95.101.67
nsa.cesnet.cz. 3 A 195.113.144.228
decsys.vsb.cz. 3 A 158.196.149.9
nsa.ces.net. 3 AAAA 2001:718:1:1:0:0:144:205
nsa.ten.cz. 3 AAAA 2a02:4a8:ac24:115:0:0:101:67
nsa.cesnet.cz. 3 AAAA 2001:718:1:101:0:0:144:228
decsys.vsb.cz. 3 AAAA 2001:718:1001:149:0:0:0:9
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0