Gemfile updated but get rake aborted! undefined method `prerequisites' for nil:NilClass error
===

I'm going through RoR tutorial and when trying to run the rake db:migrate command on page 226 of the book I get the following error: 

    rake aborted!
    undefined method `prerequisites' for nil:NilClass


Here's a copy of my gemfile: 

    source 'https://rubygems.org'

    gem 'rails', '3.2.3'
    
    # Bundle edge Rails instead:
    # gem 'rails', :git => 'git://github.com/rails/rails.git'
    
    gem 'sqlite3', :require => 'sqlite3', :group => :development
    
    
    group :development do
      gem 'rspec-rails', '2.4.0'
    end
    
    group :test do
      gem 'rspec-rails', '2.4.0'
      gem 'webrat', '0.7.1'
    end
    
    
    
    # Gems used only for assets and not required
    # in production environments by default.
    group :assets do
      gem 'sass-rails',   '~> 3.2.3'
      gem 'coffee-rails', '~> 3.2.1'
    
      # See https://github.com/sstephenson/execjs#readme for more supported runtimes
      # gem 'therubyracer', :platform => :ruby
    
      gem 'uglifier', '>= 1.0.3'
    end
    
    gem 'jquery-rails'
    
    # To use ActiveModel has_secure_password
    # gem 'bcrypt-ruby', '~> 3.0.0'
    
    # To use Jbuilder templates for JSON
    # gem 'jbuilder'
    
    # Use unicorn as the app server
    # gem 'unicorn'
    
    # Deploy with Capistrano
    # gem 'capistrano'
    
    # To use debugger
    # gem 'ruby-debug19', :require => 'ruby-debug'


My gemfile is up to date and am not sure how to resolve the error..any help is appreciated!! 
