Apex domains cannot have CNAME records.
Question and response
Question
dig si21.com. NS
Answer
si21.com. 5 NS ns.kabi.si.
si21.com. 5 NS ns1.amis.net.
si21.com. 5 NS ns1.kabi.si.
si21.com. 5 NS ns1.kabi.info.
Authority
The response has no authority records.
Additional
ns.kabi.si. 5 A 212.103.146.241
ns1.amis.net. 5 A 212.18.32.5
ns1.kabi.si. 5 A 84.255.248.210
ns1.kabi.info. 5 A 85.17.207.120
ns1.amis.net. 5 AAAA 2001:15c0:ffff:f:0:0:0:5
ns1.kabi.info. 5 AAAA 2001:1af8:4100:a050:2:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0