La adición de dominio en heroku mostrar mensaje de error

la adición de dominio en heroku mostrar mensaje de error

I'll answer my own question: according to Heroku's documentation the *.herokudns 
addresses are not browsable. Instead, Heroku uses the 'Host' header from the client 
to ensure the correct application responds to a request.

The problem here was that the Host header is coming with 'www' 
from Namecheap (since I made that my canonical name -CNAME-) but I 
didn't give the 'www' to Heroku so the solution is to add the custom 
domain in Heroku with 'www'.