Apex domains cannot have CNAME records.
Question and response
Question
dig gransy.cz. NS
Answer
gransy.cz. 5 NS ns.gransy.com.
gransy.cz. 5 NS ns3.gransy.com.
gransy.cz. 5 NS ns5.gransy.com.
gransy.cz. 5 NS ns2.gransy.com.
gransy.cz. 5 NS ns4.gransy.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0