Apex domains cannot have CNAME records.
Question and response
Question
dig thinline.cz. NS
Answer
thinline.cz. 5 NS ns2.thinline.cz.
thinline.cz. 5 NS ns3.cesky-hosting.eu.
thinline.cz. 5 NS ns1.thinline.cz.
Authority
The response has no authority records.
Additional
ns1.thinline.cz. 5 A 91.239.200.243
ns2.thinline.cz. 5 A 82.100.6.2
ns3.cesky-hosting.eu. 5 A 91.239.202.18
ns1.thinline.cz. 5 AAAA 2001:67c:e94:0:1:5bef:c8f3:1
ns2.thinline.cz. 5 AAAA 2001:4de8:fa22:0:1:5264:602:1
ns3.cesky-hosting.eu. 5 AAAA 2001:67c:e94:1:1:5bef:ca12:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0