Apex domains cannot have CNAME records.
Question and response
Question
dig ess.uk.barracuda.com. NS
Answer
ess.uk.barracuda.com. 5 NS ns-215.awsdns-26.com.
ess.uk.barracuda.com. 5 NS ns-740.awsdns-28.net.
ess.uk.barracuda.com. 5 NS ns-1481.awsdns-57.org.
ess.uk.barracuda.com. 5 NS ns-1883.awsdns-43.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0