Apex domains cannot have CNAME records.
Question and response
Question
dig ti.cz. NS
Answer
ti.cz. 4 NS dns.nextra.cz.
ti.cz. 4 NS sns.nextra.cz.
Authority
The response has no authority records.
Additional
dns.nextra.cz. 4 A 193.85.1.117
sns.nextra.cz. 4 A 193.85.2.117
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0