Apex domains cannot have CNAME records.
Question and response
Question
dig flock.co. NS
Answer
flock.co. 5 NS ns-1971.awsdns-54.co.uk.
flock.co. 5 NS ns-776.awsdns-33.net.
flock.co. 5 NS ns-176.awsdns-22.com.
flock.co. 5 NS ns-1028.awsdns-00.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0