Errno::ENOENT - No such file or directory - Sinatra application image upload

I have this simple method that writes a file and returns it as part of an image upload to AWS...

def write_image_to_file(image, filename)
  file = File.new("public/images/user_images/#{filename}", "w+")
  file.write(image)
  file
end

This works fine in development, the image uploads correctly and i get a URL of the AWS hosted image which i store in a database. However, when deployed to heroku, I get the following error message as part of the heroku logs...

Errno::ENOENT - No such file or directory - public/images/user_images/image.jpg:

The image doesn't upload. I thought it might be something to do with the public folder so have tried adding a ./ before public but this hasn't worked either.