heroku not display custom error pages

heroku not display custom error pages

I followed the tutorial here to create custom error pages:
http://wearestac.com/blog/dynamic-error-pages-in-rails
When I got to /404, /500, and /424, I can view my custom error pages
locally. However, after deploying with Heroku, I'm finding that my custom
error pages aren't loading and, instead, the app is defaulting to the
static 404.html and 500.html pages. How can I go about fixing this error?
I also tried the fix suggested in this post, but it didn't work for me:
User-friendly error pages not displaying in production environment