Apex domains cannot have CNAME records.
Question and response
Question
dig grape.cz. NS
Answer
grape.cz. 5 NS ns1.grapesc.cz.
grape.cz. 5 NS ns2.grapesc.cz.
Authority
The response has no authority records.
Additional
ns1.grapesc.cz. 5 A 93.91.144.100
ns2.grapesc.cz. 5 A 95.168.195.65
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0