Apex domains cannot have CNAME records.
Question and response
Question
dig ry.cz. NS
Answer
ry.cz. 4 NS ns3.gransy.com.
ry.cz. 4 NS ns.gransy.com.
ry.cz. 4 NS ns5.gransy.com.
ry.cz. 4 NS ns2.gransy.com.
ry.cz. 4 NS ns4.gransy.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0