Apex domains cannot have CNAME records.
Question and response
Question
dig nic.nu. NS
Answer
nic.nu. 5 NS nsa.dnsnode.net.
nic.nu. 5 NS nsp.dnsnode.net.
nic.nu. 5 NS nsu.dnsnode.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0