Apex domains cannot have CNAME records.
Question and response
Question
dig ca-central-1.elb.amazonaws.com. NS
Answer
ca-central-1.elb.amazonaws.com. 5 NS ns-127.awsdns-15.com.
ca-central-1.elb.amazonaws.com. 5 NS ns-616.awsdns-13.net.
ca-central-1.elb.amazonaws.com. 5 NS ns-1103.awsdns-09.org.
ca-central-1.elb.amazonaws.com. 5 NS ns-1767.awsdns-28.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0