Apex domains cannot have CNAME records.
Question and response
Question
dig core.ttnet.cz. NS
Answer
core.ttnet.cz. 5 NS ns.tyhan.cz.
core.ttnet.cz. 5 NS ns.thsoft.cz.
Authority
The response has no authority records.
Additional
ns.thsoft.cz. 5 A 62.204.224.3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0