Apex domains cannot have CNAME records.
Question and response
Question
dig sg. NS
Answer
sg. 5 NS ns4.apnic.net.
sg. 5 NS dsany2.sgnic.sg.
sg. 5 NS pch.sgzones.sg.
sg. 5 NS dsany4.sgnic.sg.
sg. 5 NS dsany3.sgnic.sg.
Authority
The response has no authority records.
Additional
ns4.apnic.net. 5 A 202.12.31.53
pch.sgzones.sg. 172711 A 204.61.216.57
dsany2.sgnic.sg. 170049 A 185.159.197.170
dsany3.sgnic.sg. 164815 A 185.159.198.170
dsany4.sgnic.sg. 150928 A 45.142.220.65
ns4.apnic.net. 5 AAAA 2001:dd8:12:0:0:0:0:53
pch.sgzones.sg. 172711 AAAA 2001:500:14:6057:ad:0:0:1
dsany2.sgnic.sg. 170049 AAAA 2620:10a:80aa:0:0:0:0:170
dsany3.sgnic.sg. 164815 AAAA 2620:10a:80ab:0:0:0:0:170
dsany4.sgnic.sg. 150928 AAAA 2a0e:dbc0:1:0:0:0:0:65
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0