Apex domains cannot have CNAME records.
Question and response
Question
dig cust.avonet.cz. NS
Answer
cust.avonet.cz. 2 NS tns.avonet.cz.
cust.avonet.cz. 2 NS sns.avonet.cz.
cust.avonet.cz. 2 NS ns.avonet.cz.
Authority
The response has no authority records.
Additional
ns.avonet.cz. 2 A 217.112.162.34
sns.avonet.cz. 2 A 93.91.154.61
tns.avonet.cz. 2 A 217.112.160.5
ns.avonet.cz. 2 AAAA 2a00:6500:0:11:0:0:0:a
sns.avonet.cz. 2 AAAA 2a02:17a0:fff1:2003:0:0:0:a
tns.avonet.cz. 2 AAAA 2a00:6500:0:f2:0:0:0:c
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0