Apex domains cannot have CNAME records.
Question and response
Question
dig nu. NS
Answer
nu. 5 NS z.ns.nu.
nu. 5 NS a.ns.nu.
nu. 5 NS c.ns.nu.
nu. 5 NS d.ns.nu.
nu. 5 NS m.ns.nu.
nu. 5 NS y.ns.nu.
Authority
The response has no authority records.
Additional
a.ns.nu. 55531 A 194.146.106.22
c.ns.nu. 55531 A 192.36.144.107
d.ns.nu. 55531 A 213.108.25.4
m.ns.nu. 55531 A 194.0.11.112
y.ns.nu. 55531 A 185.159.197.150
z.ns.nu. 55531 A 185.159.198.150
a.ns.nu. 55531 AAAA 2001:67c:1010:5:0:0:0:53
c.ns.nu. 55531 AAAA 2a01:3f0:0:301:0:0:0:53
d.ns.nu. 55531 AAAA 2001:67c:124c:e000:0:0:0:4
m.ns.nu. 55531 AAAA 2001:678:e:112:0:0:0:53
y.ns.nu. 55531 AAAA 2620:10a:80aa:0:0:0:0:150
z.ns.nu. 55531 AAAA 2620:10a:80ab:0:0:0:0:150
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0