Apex domains cannot have CNAME records.
Question and response
Question
dig nic.is. NS
Answer
nic.is. 5 NS tg-secondary01.isnic.is.
nic.is. 5 NS sec1.rcode0.net.
nic.is. 5 NS nsp.dnsnode.net.
nic.is. 5 NS ht-secondary01.isnic.is.
Authority
The response has no authority records.
Additional
sec1.rcode0.net. 5 A 192.174.68.100
ht-secondary01.isnic.is. 5 A 185.93.156.39
tg-secondary01.isnic.is. 5 A 185.93.156.162
sec1.rcode0.net. 5 AAAA 2001:67c:1bc:0:0:0:0:100
ht-secondary01.isnic.is. 5 AAAA 2001:67c:6c:56:0:0:0:39
tg-secondary01.isnic.is. 5 AAAA 2001:67c:6c:f056:0:0:0:162
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0