Apex domains cannot have CNAME records.
Question and response
Question
dig singtel.com. NS
Answer
singtel.com. 5 NS a12-67.akam.net.
singtel.com. 5 NS a1-158.akam.net.
singtel.com. 5 NS a5-65.akam.net.
singtel.com. 5 NS a4-67.akam.net.
singtel.com. 5 NS a7-67.akam.net.
singtel.com. 5 NS a10-67.akam.net.
Authority
The response has no authority records.
Additional
a4-67.akam.net. 5 A 72.246.46.67
a12-67.akam.net. 5 A 184.26.160.67
a4-67.akam.net. 5 AAAA 2600:1480:9000:0:0:0:0:43
a12-67.akam.net. 5 AAAA 2600:1480:f000:0:0:0:0:43
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0