Apex domains cannot have CNAME records.
Question and response
Question
dig cust.asergo.com. NS
Answer
cust.asergo.com. 4 NS ns1.asergo.com.
cust.asergo.com. 4 NS ns2.asergo.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0