Apex domains cannot have CNAME records.
Question and response
Question
dig r.cloudfront.net. NS
Answer
r.cloudfront.net. 3 NS ns-954.awsdns-55.net.
r.cloudfront.net. 3 NS ns-1622.awsdns-10.co.uk.
r.cloudfront.net. 3 NS ns-279.awsdns-34.com.
r.cloudfront.net. 3 NS ns-1204.awsdns-22.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0