Apex domains cannot have CNAME records.
Question and response
Question
dig 24.73.203.ptr.twnic.net.tw. NS
Answer
24.73.203.ptr.twnic.net.tw. 5 NS ptr2.twnic.net.tw.
24.73.203.ptr.twnic.net.tw. 5 NS ptr1.twnic.net.tw.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0