Apex domains cannot have CNAME records.
Question and response
Question
dig prod.hydra.sophos.com. NS
Answer
prod.hydra.sophos.com. 5 NS ns-494.awsdns-61.com.
prod.hydra.sophos.com. 5 NS ns-1793.awsdns-32.co.uk.
prod.hydra.sophos.com. 5 NS ns-1043.awsdns-02.org.
prod.hydra.sophos.com. 5 NS ns-684.awsdns-21.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0