Apex domains cannot have CNAME records.
Question and response
Question
dig techspace.cz. NS
Answer
techspace.cz. 3 NS sns.techspace.cz.
techspace.cz. 3 NS ns.techspace.cz.
Authority
The response has no authority records.
Additional
ns.techspace.cz. 3 A 81.0.217.191
sns.techspace.cz. 3 A 81.0.217.236
ns.techspace.cz. 1 AAAA 2001:1528:224:ffff:0:0:0:191
sns.techspace.cz. 1 AAAA 2001:1528:224:ffff:0:0:0:236
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0