Apex domains cannot have CNAME records.
Question and response
Question
dig zelenadata.cz. NS
Answer
zelenadata.cz. 5 NS inet.faster.cz.
zelenadata.cz. 5 NS inet.zelenadata.cz.
Authority
The response has no authority records.
Additional
inet.faster.cz. 5 A 81.19.0.66
inet.zelenadata.cz. 5 A 81.19.10.33
inet.faster.cz. 5 AAAA 2a02:e98:0:3:0:0:0:53
inet.zelenadata.cz. 5 AAAA 2a02:e98:0:3:0:0:2:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0