Apex domains cannot have CNAME records.
Question and response
Question
dig greenhouse.io. NS
Answer
greenhouse.io. 5 NS ns-491.awsdns-61.com.
greenhouse.io. 5 NS ns-1030.awsdns-00.org.
greenhouse.io. 5 NS ns-979.awsdns-58.net.
greenhouse.io. 5 NS ns-1915.awsdns-47.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0