Apex domains cannot have CNAME records.
Question and response
Question
dig myvouchercodes.co.uk. NS
Answer
myvouchercodes.co.uk. 5 NS ns-1192.awsdns-21.org.
myvouchercodes.co.uk. 5 NS ns-1772.awsdns-29.co.uk.
myvouchercodes.co.uk. 5 NS ns-45.awsdns-05.com.
myvouchercodes.co.uk. 5 NS ns-582.awsdns-08.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0