Apex domains cannot have CNAME records.
Question and response
Question
dig sigsync.com. NS
Answer
sigsync.com. 5 NS ns-1325.awsdns-37.org.
sigsync.com. 5 NS ns-228.awsdns-28.com.
sigsync.com. 5 NS ns-795.awsdns-35.net.
sigsync.com. 5 NS ns-1795.awsdns-32.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0