Apex domains cannot have CNAME records.
Question and response
Question
dig ignum.com. NS
Answer
ignum.com. 5 NS ns2.ignum.cz.
ignum.com. 5 NS ns1.ignum.com.
Authority
The response has no authority records.
Additional
ns1.ignum.com. 5 A 185.65.223.35
ns2.ignum.cz. 5 A 217.31.48.201
ns1.ignum.com. 5 AAAA 2a00:1e40:a:53:0:0:0:5
ns2.ignum.cz. 5 AAAA 2001:1ab0:7e1e:2:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0