Apex domains cannot have CNAME records.
Question and response
Question
dig pdx.net.uk. NS
Answer
pdx.net.uk. 5 NS orbit.ns.pdxdns.org.
pdx.net.uk. 5 NS nova.ns.pdxdns.co.uk.
pdx.net.uk. 5 NS nebula.ns.pdxdns.com.
pdx.net.uk. 5 NS apollo.ns.pdxdns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0