Apex domains cannot have CNAME records.
Question and response
Question
dig sdi.trendmicro.com. NS
Answer
sdi.trendmicro.com. 1 NS a20-67.akam.net.
sdi.trendmicro.com. 1 NS a1-163.akam.net.
sdi.trendmicro.com. 1 NS a8-65.akam.net.
sdi.trendmicro.com. 1 NS a5-64.akam.net.
sdi.trendmicro.com. 1 NS a9-66.akam.net.
sdi.trendmicro.com. 1 NS a12-66.akam.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0