Apex domains cannot have CNAME records.
Question and response
Question
dig thnic.net. NS
Answer
thnic.net. 5 NS ns.thnic.net.
thnic.net. 5 NS ns-a.thnic.co.th.
thnic.net. 5 NS ns-b.thnic.co.th.
Authority
The response has no authority records.
Additional
ns.thnic.net. 5 A 202.28.0.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0