Apex domains cannot have CNAME records.
Question and response
Question
dig 117.203.in-addr.arpa. NS
Answer
117.203.in-addr.arpa. 4 NS secdns2.starhub.net.sg.
117.203.in-addr.arpa. 4 NS pricus.starhub.net.sg.
117.203.in-addr.arpa. 4 NS 203.117.203.in-addr.arpa.
Authority
The response has no authority records.
Additional
pricus.starhub.net.sg. 4 A 203.116.1.93
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0