Apex domains cannot have CNAME records.
Question and response
Question
dig vast-mesa-1544.herokuspace.com. NS
Answer
vast-mesa-1544.herokuspace.com. 5 NS ns-1078.awsdns-06.org.
vast-mesa-1544.herokuspace.com. 5 NS ns-393.awsdns-49.com.
vast-mesa-1544.herokuspace.com. 5 NS ns-668.awsdns-19.net.
vast-mesa-1544.herokuspace.com. 5 NS ns-1584.awsdns-06.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0