Application error uploading file to AWS using carrierwave and fog - heroku deployed app

I've set heroku config vars with AWS credentials using:

heroku config:set S3_ACCESS_KEY=<....>
heroku config:set S3_SECRET_KEY=<....>
heroku config:set S3_BUCKET=<....>

and verified them on heroku:

heroku run rails console

CarrierWave.configure do |config|
  puts config.fog_directory
  puts config.fog_credentials
end

On my config/initializers/carrier_wave.rb file, I've got:

if Rails.env.production?
  CarrierWave.configure do |config|
    config.fog_credentials = {
    :provider              => 'AWS',
    :aws_access_key_id     => ENV['S3_ACCESS_KEY'],
    :aws_secret_access_key => ENV['S3_SECRET_KEY']
  }
  config.fog_directory     =  ENV['S3_BUCKET']
  end
end

On app/uploaders/picture_uploader.rb file I've got:

  if Rails.env.production?
    storage :fog
  else
    storage :file
  end

When deploying the app on heroku everything seems to be ok but trying upload a file leads to an Application Error page... Does this have to do with AWS rather than my code?