Apex domains cannot have CNAME records.
Question and response
Question
dig pp-dns.com. NS
Answer
pp-dns.com. 1 NS ns-144.awsdns-18.com.
pp-dns.com. 1 NS ns-1278.awsdns-31.org.
pp-dns.com. 1 NS ns-841.awsdns-41.net.
pp-dns.com. 1 NS ns-1676.awsdns-17.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0