Apex domains cannot have CNAME records.
Question and response
Question
dig rea.o2.cz. NS
Answer
rea.o2.cz. 4 NS dns.iol.cz.
rea.o2.cz. 4 NS ns2.tel.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0