Apex domains cannot have CNAME records.
Question and response
Question
dig cpu.co.uk. NS
Answer
cpu.co.uk. 5 NS ns1049.ui-dns.org.
cpu.co.uk. 5 NS ns1018.ui-dns.de.
cpu.co.uk. 5 NS ns1024.ui-dns.com.
cpu.co.uk. 5 NS ns1075.ui-dns.biz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0