Apex domains cannot have CNAME records.
Question and response
Question
dig is. NS
Answer
is. 5 NS e.nic.is.
is. 5 NS f.nic.is.
is. 5 NS a.nic.is.
is. 5 NS c.nic.is.
is. 5 NS d.nic.is.
Authority
The response has no authority records.
Additional
a.nic.is. 172598 A 185.93.156.10
c.nic.is. 19161 A 204.61.216.116
d.nic.is. 19161 A 194.146.106.58
e.nic.is. 19161 A 194.0.25.41
f.nic.is. 134176 A 185.93.157.22
a.nic.is. 172598 AAAA 2001:67c:6c:56:0:0:0:10
c.nic.is. 19161 AAAA 2001:500:14:6116:ad:0:0:1
d.nic.is. 19161 AAAA 2001:67c:1010:14:0:0:0:53
e.nic.is. 19161 AAAA 2001:678:20:0:0:0:0:41
f.nic.is. 134176 AAAA 2001:67c:6c:1000:0:0:0:22
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0