Apex domains cannot have CNAME records.
Question and response
Question
dig eml100syd.ctr.sophos.com. NS
Answer
eml100syd.ctr.sophos.com. 5 NS ns-85.awsdns-10.com.
eml100syd.ctr.sophos.com. 5 NS ns-1876.awsdns-42.co.uk.
eml100syd.ctr.sophos.com. 5 NS ns-1348.awsdns-40.org.
eml100syd.ctr.sophos.com. 5 NS ns-996.awsdns-60.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0