Apex domains cannot have CNAME records.
Question and response
Question
dig lk. NS
Answer
lk. 5 NS t.nic.lk.
lk. 5 NS h.nic.lk.
lk. 5 NS d.nic.lk.
lk. 5 NS m.nic.lk.
lk. 5 NS p.nic.lk.
lk. 5 NS pendragon.cs.purdue.edu.
lk. 5 NS ns1.ac.lk.
lk. 5 NS z.nic.lk.
Authority
The response has no authority records.
Additional
d.nic.lk. 148348 A 123.231.6.18
h.nic.lk. 98465 A 43.252.12.206
m.nic.lk. 98465 A 202.129.235.229
p.nic.lk. 164253 A 204.61.216.27
t.nic.lk. 98465 A 203.94.66.129
z.nic.lk. 98465 A 185.38.108.108
d.nic.lk. 148348 AAAA 2402:4000:479:c5:0:0:0:a
h.nic.lk. 98465 AAAA 2404:8a00:0:a00:0:0:0:4005
m.nic.lk. 98465 AAAA 2407:c00:21:8:202:129:235:229
p.nic.lk. 164253 AAAA 2001:500:14:6027:ad:0:0:1
t.nic.lk. 98465 AAAA 2402:d000:100c:0:0:0:0:129
z.nic.lk. 98465 AAAA 2a00:fea0:dead:0:0:0:0:beef
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0