Apex domains cannot have CNAME records.
Question and response
Question
dig aspi.cz. NS
Answer
aspi.cz. 5 NS edns9.ultradns.biz.
aspi.cz. 5 NS edns9.ultradns.net.
aspi.cz. 5 NS edns9.ultradns.com.
aspi.cz. 5 NS edns9.ultradns.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0