Apex domains cannot have CNAME records.
Question and response
Question
dig anycastdns.cz. NS
Answer
anycastdns.cz. 5 NS ns2.anycastdns.cz.
anycastdns.cz. 5 NS ns1.anycastdns.cz.
anycastdns.cz. 5 NS sec2.rcode0.net.
anycastdns.cz. 5 NS sec1.rcode0.net.
Authority
The response has no authority records.
Additional
ns1.anycastdns.cz. 5 A 185.38.108.108
ns2.anycastdns.cz. 5 A 185.28.194.194
sec1.rcode0.net. 4 A 192.174.68.100
sec2.rcode0.net. 4 A 176.97.158.100
ns1.anycastdns.cz. 5 AAAA 2a00:fea0:dead:0:0:0:0:beef
sec1.rcode0.net. 4 AAAA 2001:67c:1bc:0:0:0:0:100
sec2.rcode0.net. 4 AAAA 2001:67c:10b8:0:0:0:0:100
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0