Apex domains cannot have CNAME records.
Question and response
Question
dig static.wolfnet.cz. NS
Answer
static.wolfnet.cz. 5 NS ns1.wolfnet.cz.
static.wolfnet.cz. 5 NS ns2.wolfnet.cz.
Authority
The response has no authority records.
Additional
ns1.wolfnet.cz. 5 A 185.73.101.2
ns2.wolfnet.cz. 5 A 185.73.108.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0