Apex domains cannot have CNAME records.
Question and response
Question
dig sunet.se. NS
Answer
sunet.se. 5 NS ns1.sunet.se.
sunet.se. 5 NS sunic.sunet.se.
sunet.se. 5 NS server.nordu.net.
sunet.se. 5 NS b.ns.kth.se.
Authority
The response has no authority records.
Additional
b.ns.kth.se. 5 A 130.237.72.250
ns1.sunet.se. 5 A 89.47.185.240
sunic.sunet.se. 5 A 192.36.125.2
server.nordu.net. 5 A 193.10.252.19
b.ns.kth.se. 5 AAAA 2001:6b0:1:0:0:0:0:250
ns1.sunet.se. 5 AAAA 2001:6b0:5a:4020:0:0:0:384
sunic.sunet.se. 5 AAAA 2001:6b0:7:0:0:0:0:2
server.nordu.net. 5 AAAA 2001:948:4:2:0:0:0:19
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0