Apex domains cannot have CNAME records.
Question and response
Question
dig icu. NS
Answer
icu. 3 NS b.nic.icu.
icu. 3 NS a.nic.icu.
icu. 3 NS d.nic.icu.
icu. 3 NS c.nic.icu.
Authority
The response has no authority records.
Additional
a.nic.icu. 134358 A 194.169.218.108
b.nic.icu. 134358 A 185.24.64.108
c.nic.icu. 134358 A 212.18.248.108
d.nic.icu. 134358 A 212.18.249.108
a.nic.icu. 134358 AAAA 2001:67c:13cc:0:0:0:1:108
b.nic.icu. 134358 AAAA 2a04:2b00:13cc:0:0:0:1:108
c.nic.icu. 134358 AAAA 2a04:2b00:13ee:0:0:0:0:108
d.nic.icu. 134358 AAAA 2a04:2b00:13ff:0:0:0:0:108
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0