Should create a TXT record indicating that it is really an ALIAS

[rdlugosz]

This is something DNSimple does when you use their ALIAS record type. I suspect Heroku looks for this as confirmation that you're not mapping to a naked domain. Should replicate.

$  dig example.com

;; QUESTION SECTION:
;example.com.                        IN      A

;; ANSWER SECTION:
example.com.         598     IN      A       54.243.193.13
example.com.         598     IN      A       23.23.162.212
example.com.         598     IN      A       23.21.148.128


$  dig example.com TXT

;; QUESTION SECTION:
;example.com.                        IN      TXT

;; ANSWER SECTION:
example.com.         600     IN      TXT     "ALIAS for foobar-7052.herokussl.com"