Apex domains cannot have CNAME records.
Question and response
Question
dig ji.cz. NS
Answer
ji.cz. 5 NS ins2.ji.cz.
ji.cz. 5 NS ins1.ji.cz.
Authority
The response has no authority records.
Additional
ins1.ji.cz. 5 A 213.226.224.50
ins2.ji.cz. 5 A 213.226.224.51
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0