Apex domains cannot have CNAME records.
Question and response
Question
dig internext.cz. NS
Answer
internext.cz. 5 NS ns2.inext.cz.
internext.cz. 5 NS ns1.inext.cz.
Authority
The response has no authority records.
Additional
ns1.inext.cz. 5 A 212.111.0.11
ns2.inext.cz. 5 A 194.213.32.237
ns1.inext.cz. 5 AAAA 2a00:e580:4:0:0:0:0:11
ns2.inext.cz. 5 AAAA 2001:af0:100:0:0:0:0:c
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0