Topic: problem render erb

Hello people! Need help. I moved the rails-application on a different server. Now the application is not running.

def index
    @books = Book.all
    respond_to do |format|
      format.html  # index.html.erb 
      format.xml { render xml: @books }

if format = html,  in the log I see the error:
NoMethodError (undefined method `empty?' for nil:NilClass):
  app/controllers/books_controller.rb:14:in `index'
  lib/content_length_fix.rb:8:in `call'

what happens? I just moved the application.