Apex domains cannot have CNAME records.
Question and response
Question
dig iras.gov.sg. NS
Answer
iras.gov.sg. 5 NS a3-67.akam.net.
iras.gov.sg. 5 NS a8-64.akam.net.
iras.gov.sg. 5 NS ns12.gdnsdef.com.
iras.gov.sg. 5 NS ns12.gdnsec.com.
iras.gov.sg. 5 NS ns10.gdnsdef.com.
iras.gov.sg. 5 NS a20-66.akam.net.
iras.gov.sg. 5 NS ns3.gdnsec.com.
iras.gov.sg. 5 NS a1-6.akam.net.
iras.gov.sg. 5 NS ns9.gdnsec.com.
iras.gov.sg. 5 NS a2-65.akam.net.
iras.gov.sg. 5 NS a9-65.akam.net.
iras.gov.sg. 5 NS ns7.gdnsdef.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0