Apex domains cannot have CNAME records.
Question and response
Question
dig coresite.com. NS
Answer
coresite.com. 5 NS ns-1194.awsdns-21.org.
coresite.com. 5 NS ns-437.awsdns-54.com.
coresite.com. 5 NS ns-874.awsdns-45.net.
coresite.com. 5 NS ns-1952.awsdns-52.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0