Apex domains cannot have CNAME records.
Question and response
Question
dig sophos.com. NS
Answer
sophos.com. 5 NS a14-67.akam.net.
sophos.com. 5 NS a9-65.akam.net.
sophos.com. 5 NS a1-100.akam.net.
sophos.com. 5 NS a10-65.akam.net.
sophos.com. 5 NS a11-66.akam.net.
sophos.com. 5 NS a18-64.akam.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0