Apex domains cannot have CNAME records.
Question and response
Question
dig hk. NS
Answer
hk. 5 NS x.hkirc.net.hk.
hk. 5 NS d.hkirc.net.hk.
hk. 5 NS u.hkirc.net.hk.
hk. 5 NS t.hkirc.net.hk.
hk. 5 NS v.hkirc.net.hk.
hk. 5 NS m.hkirc.net.hk.
hk. 5 NS c.hkirc.net.hk.
hk. 5 NS y.hkirc.net.hk.
hk. 5 NS z.hkirc.net.hk.
Authority
The response has no authority records.
Additional
c.hkirc.net.hk. 64404 A 203.119.2.218
d.hkirc.net.hk. 71294 A 203.119.87.218
m.hkirc.net.hk. 130353 A 125.208.49.10
t.hkirc.net.hk. 130353 A 202.12.31.53
u.hkirc.net.hk. 130353 A 210.201.138.58
v.hkirc.net.hk. 130353 A 204.61.216.46
x.hkirc.net.hk. 130353 A 202.45.188.39
y.hkirc.net.hk. 130353 A 137.189.6.21
z.hkirc.net.hk. 156828 A 194.146.106.70
c.hkirc.net.hk. 64404 AAAA 2001:dca:4000:0:0:0:cb77:2da
d.hkirc.net.hk. 71294 AAAA 2001:dca:2000:0:0:0:cb77:57da
m.hkirc.net.hk. 130353 AAAA 2001:dc7:feff:0:0:0:0:10
t.hkirc.net.hk. 130353 AAAA 2001:dd8:12:0:0:0:0:53
u.hkirc.net.hk. 130353 AAAA 2404:0:10a0:0:0:0:0:58
v.hkirc.net.hk. 130353 AAAA 2001:500:14:6046:ad:0:0:1
x.hkirc.net.hk. 130353 AAAA 2405:3001:1:3a:0:0:0:27
y.hkirc.net.hk. 130353 AAAA 2405:3000:3:6:0:0:0:15
z.hkirc.net.hk. 156828 AAAA 2001:67c:1010:17:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0