Apex domains cannot have CNAME records.
Question and response
Question
dig swbc.com. NS
Answer
swbc.com. 5 NS ns-1289.awsdns-33.org.
swbc.com. 5 NS ns-58.awsdns-07.com.
swbc.com. 5 NS ns-570.awsdns-07.net.
swbc.com. 5 NS ns-1872.awsdns-42.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0