Hidden attribute not populating field in form for Rails 3.2

I'm implementing an invitation system and I want the new user form to pre-populate the user's email address in the email address field on the form (eventually, I will refactor this so it's not a form_field), so that the user doesn't have to type in all their information, just enter a password.

I have created the getter/setter methods in the users.rb model like this:

  def invitation_token
    invitation.invitation_token if invitation
  end
  def invitation_token=(invitation_token)
    self.invitation = Invitation.find_by_invitation_token(invitation_token)
  end

INVITATION MODEL

class Invitation < ActiveRecord::Base

  #--== ASSOCIATIONS
  belongs_to :sender, :class_name => 'User'
  has_one :recipient, :class_name => 'User'
  #--== CALLBACKS
  before_create :generate_token
  before_create :recipient_is_not_registered
  before_create :decrement_sender_count, :if => :sender
  #--== VALIDATIONS
  validates_presence_of :recipient_email
  #validate :recipient_is_not_registered
  validate :sender_has_invitations, :if => :sender
  #--== METHODS
  private
    def recipient_is_not_registered
      if User.find_by_email(recipient_email)
        false
      else
        true
      end
    end

    def sender_has_invitations
      unless sender.invitation_limit > 0
        redirect_to root_url
      end
    end

    def generate_token  #TODO: MOVE to lib/generate_token.rb
      self.invitation_token = Digest::SHA1.hexdigest([Time.now, rand].join)
    end

    def decrement_sender_count
      sender.decrement! :invitation_limit
    end

end

USER CONTROLLER

class UsersController < ApplicationController
  def new
    @user = User.new(:invitation_token => params[:invitation_token])
    @user.email = @user.invitation.recipient_email if @user.invitation
  end

  def create
    @user = User.new(user_params)
    if @user.save
      session[:user_id] = @user.id
      redirect_to root_url, notice: "Thank you for signing up!"
    else
      render "new"
    end
  end

 ...

  def user_params
    params.require(:user).permit(:email, :password, :password_confirmation, :admin)
  end
end

views/users/_form.html.erb

<%= form_for @user do |f| %>  

  <%= f.hidden_field :invitation_token %>

  <div class="field">
    <%= f.label :email %><br />
    <%= f.text_field :email %>
  </div>
  <div class="field">
    <%= f.label :password %><br />
    <%= f.password_field :password %>
  </div>
  <div class="field">
    <%= f.label :password_confirmation %><br />
    <%= f.password_field :password_confirmation %>
  </div>
  <div class="field">
    <%= f.check_box :admin %>
    <%= f.label :admin %>
  </div>
  <div class="actions"><%= f.submit %></div>
<% end %>

I was following Ryan Bates' RC#124 - Beta Invitations, and got stuck here. His code doesn't produce the error, so I should mention that this is a Rails 3.2.18 app.

When I reload the form, the user's email isn't populated in the form. The relevant log shows:

Started GET "/signup.914823d28d07b747213ec3de47f89ad537169e34" for 127.0.0.1 
at 2016-04-30 20:24:47 -0600
Processing by UsersController#new as 
  User Load (1.0ms)  SELECT "users".* FROM "users" WHERE "users"."auth_token" = 'rOHiKmDcceytxi_t151YIQ' LIMIT 1
  Invitation Load (0.0ms)  SELECT "invitations".* FROM "invitations" WHERE "invitations"."invitation_token" IS NULL LIMIT 1
  Rendered users/_form.html.erb (5.0ms)
  Rendered users/new.html.erb within layouts/application (6.0ms)
Completed 200 OK in 102.0ms (Views: 25.0ms | ActiveRecord: 3.0ms)

So it appears that the invitation_token isn't being passed in, since the log shows it is NULL.

I have gone over the RC code from top to bottom and can't find out why it's not being passed.

Any help would be appreciated. Thanks.

UPDATE: The output from the view source is:

<input id="user_invitation_token" name="user[invitation_token]" type="hidden" />, so it's not being passed along.