Apex domains cannot have CNAME records.
Question and response
Question
dig bcom.cz. NS
Answer
bcom.cz. 5 NS ns.bcom.cz.
bcom.cz. 5 NS ns.webx.cz.
Authority
The response has no authority records.
Additional
ns.bcom.cz. 5 A 89.185.253.134
ns.webx.cz. 5 A 82.208.12.1
ns.bcom.cz. 5 AAAA 2a01:430:208:8659:0:0:0:10
ns.webx.cz. 5 AAAA 2001:1528:246:0:ff52:d00c:1ff:0
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0