ext-cust.squarespace.com has no CNAME records.
Question and response
Question
dig ext-cust.squarespace.com. NS
Answer
The response has no answer records.
Authority
squarespace.com. 780 SOA dns1.p06.nsone.net. dns-admin.squarespace.com. 1663787404 3600 600 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0