Apex domains cannot have CNAME records.
Question and response
Question
dig crowncastle.com. NS
Answer
crowncastle.com. 5 NS ns-489.awsdns-61.com.
crowncastle.com. 5 NS ns-1209.awsdns-23.org.
crowncastle.com. 5 NS ns-757.awsdns-30.net.
crowncastle.com. 5 NS ns-1902.awsdns-45.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0