Apex domains cannot have CNAME records.
Question and response
Question
dig umbler.co.uk. NS
Answer
umbler.co.uk. 5 NS ns-1673.awsdns-17.co.uk.
umbler.co.uk. 5 NS ns-346.awsdns-43.com.
umbler.co.uk. 5 NS ns-1003.awsdns-61.net.
umbler.co.uk. 5 NS ns-1387.awsdns-45.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0