Apex domains cannot have CNAME records.
Question and response
Question
dig sailthru-stage.cloud. NS
Answer
sailthru-stage.cloud. 5 NS ns-1118.awsdns-11.org.
sailthru-stage.cloud. 5 NS ns-400.awsdns-50.com.
sailthru-stage.cloud. 5 NS ns-729.awsdns-27.net.
sailthru-stage.cloud. 5 NS ns-1692.awsdns-19.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0