Apex domains cannot have CNAME records.
Question and response
Question
dig nethost.cz. NS
Answer
nethost.cz. 4 NS tns.nethost.eu.
nethost.cz. 4 NS dns.nethost.cz.
nethost.cz. 4 NS pns.nethost.cz.
nethost.cz. 4 NS sns.nethost.sk.
Authority
The response has no authority records.
Additional
dns.nethost.cz. 4 A 81.0.235.1
pns.nethost.cz. 4 A 217.11.242.84
sns.nethost.sk. 4 A 93.185.100.244
dns.nethost.cz. 4 AAAA 2001:1528:132:0:0:0:0:60
sns.nethost.sk. 4 AAAA 2a00:1ed0:46:200:0:0:1212:247
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0