Apex domains cannot have CNAME records.
Question and response
Question
dig tw1.com. NS
Answer
tw1.com. 2 NS ns1.tw1.com.
tw1.com. 2 NS ns2.tw1.com.
Authority
The response has no authority records.
Additional
ns1.tw1.com. 2 A 221.132.112.8
ns2.tw1.com. 2 A 221.132.112.9
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0