Apex domains cannot have CNAME records.
Question and response
Question
dig t-mobile.cz. NS
Answer
t-mobile.cz. 5 NS tmobile2.ns.nic.cz.
t-mobile.cz. 5 NS tmobile1.ns.nic.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0