Apex domains cannot have CNAME records.
Question and response
Question
dig a.cloudfilter.net. NS
Answer
a.cloudfilter.net. 5 NS ns-1823.awsdns-35.co.uk.
a.cloudfilter.net. 5 NS ns-439.awsdns-54.com.
a.cloudfilter.net. 5 NS ns-1008.awsdns-62.net.
a.cloudfilter.net. 5 NS ns-1469.awsdns-55.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0