500 Error

I'm getting a 500 error.

However, through debugging I can see it is properly setting the root directory as public_html, it is loading the index.php in the root directory, it is loading the config.php and database.php, and it is loading the routes.php.

In my routes.php I have:
$route['default_controller'] = "home";


I am trying to load on a heroku staging server using the heroku-php-apache2 buildpack.

When I go to the homepage, the root, I'm getting a 500 error and it is not loading the home/index controller method like it is supposed to. When I reference a css file, such as https://staging-app/css/grid.css, it does load that.

In case needed, my .htaccess is

PHP Code:

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteBase /
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^(.*)$ index.php?/$1 [L]
</IfModule>

<IfModule !mod_rewrite.c>
    ErrorDocument 404 /index.php
</IfModule> 


Any ideas on where I could be going wrong to get a 500 error or how I can track it down?