Apex domains cannot have CNAME records.
Question and response
Question
dig tenuk.co.uk. NS
Answer
tenuk.co.uk. 5 NS ns-994.awsdns-60.net.
tenuk.co.uk. 5 NS ns-1151.awsdns-15.org.
tenuk.co.uk. 5 NS ns-1840.awsdns-38.co.uk.
tenuk.co.uk. 5 NS ns-248.awsdns-31.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0