user model associated with comment defined in plugin has no methods?

Question

I'm using the acts_as_commentable_with_threading plugin. It comes with a comment model in comment.rb:

class Comment < ActiveRecord::Base
  ....

  # NOTE: Comments belong to a user
  belongs_to :user

  ....

However, when I have a Comment object called comment, I can't call comment.user as I would have expected to be able to. In particular, I end up with this error

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.include?

and following the stack trace reveals that it occurs at this line:

  def thumb_image_for(comment)
    if comment.user.facebook_uid  #### ERROR ON THIS LINE
    ...

I tried amending the definition to this:

  #this method was called as thumb_image_for(comment.user)
  def thumb_image_for(comment)
    puts comment.to_yaml
    puts "\n\nUser is: #{comment.user}\n\n"
    puts "\n\nUser id is: #{comment.user_id}\n\n"
    puts "\n\nUser yaml on find is: #{User.find(comment.user_id).to_yaml}\n\n"
    puts "\n\nUser yaml is: #{comment.user.to_yaml}\n\n"    #### ERROR ON THIS LINE
    if comment.user.facebook_uid
    ...

And here's the output. As you can see, the user_id is valid because I can look up the user and print out his data, I just can't get it straight from comment.user. What's going on?

    --- !ruby/object:Comment 
    attributes: 
      created_at: 2010-08-11 20:40:42
      body: asdfasdfasdf
      title: ""
      commentable_type: Playlist
      updated_at: 2010-08-11 20:40:42
      flag: 
      commentable_id: 10
      lft: 1
      subject: ""
      id: 102
      user_id: 0
      parent_id: 
      visitor_name: 
      rgt: 2
    attributes_cache: {}



    User is: #<User:0x105dbb540>



    User id is: 0



    User yaml on find is: --- !ruby/object:User 
    attributes: 
      name: user1281557087
      last_request_at: 2010-08-11 21:03:49
      created_at: 2010-08-11 20:04:47
      facebook_uid: 1326120171
      updated_at: 2010-08-11 21:03:49
      moderation_reject_correct: 0
      moderation_approve_level: 0
      gender: 0
      current_login_ip: 
      openid_identifier: 
      id: 0
      moderation_approve_count: 0
      current_login_at: 
      one_liner: 
      moderation_reject_level: 0
      birthday: 
      facebook_session_key: 
      login_count: 0
      locale: en_US
      last_login_ip: 
      moderation_approve_correct: 0
      email_autoset: f
      last_login_at: 
      login: 
      moderation_reject_count: 0
      email: 
    attributes_cache: {}

ActionView::TemplateError (You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.include?) on line #5 of app/views/shared/_comment.html.haml:
2:  =link_to "", "", :name => "#{comment_counter}"
3:  .comment-info
4:      -puts comment.to_yaml
5:      = thumb_image_for_user_if_non_nil(comment)
6:      -#%img.avatar{:alt => "", :height => "40", :src => "images/gravatar.jpg", :width => "40"}/
7:      %cite
8:          =link_to_user_if_non_nil(comment)

    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/yaml.rb:389:in `quick_emit'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/yaml/rubytypes.rb:15:in `to_yaml'
    app/helpers/playlists_helper.rb:33:in `thumb_image_for_user_if_non_nil'
    app/views/shared/_comment.html.haml:5:in `_run_haml_app47views47shared47_comment46html46haml_locals_comment_comment_counter_comment_ids_object'
    haml (3.0.15) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    haml (3.0.15) lib/haml/helpers.rb:90:in `non_haml'
    haml (3.0.15) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    app/views/shared/_comments.html.haml:6:in `_run_haml_app47views47shared47_comments46html46haml_locals_comments_object'
    haml (3.0.15) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    haml (3.0.15) lib/haml/helpers.rb:90:in `non_haml'
    haml (3.0.15) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    app/views/shared/_playlist_detail.html.haml:44:in `_run_haml_app47views47shared47_playlist_detail46html46haml_locals_object_playlist_playlist_detail'
    haml (3.0.15) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    haml (3.0.15) lib/haml/helpers.rb:90:in `non_haml'
    haml (3.0.15) rails/./lib/haml/helpers/action_view_mods.rb:11:in `render'
    app/views/playlists/show.html.haml:1:in `_run_haml_app47views47playlists47show46html46haml'
    haml (3.0.15) rails/./lib/haml/helpers/action_view_mods.rb:13:in `render'
    haml (3.0.15) rails/./lib/haml/helpers/action_view_mods.rb:13:in `render'
    app/controllers/playlists_controller.rb:111:in `show'
    haml (3.0.15) rails/./lib/sass/plugin/rack.rb:41:in `call'
    haml (3.0.15) rails/./lib/sass/plugin/rack.rb:41:in `call'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/server.rb:162:in `start'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/server.rb:95:in `start'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/server.rb:92:in `each'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/server.rb:92:in `start'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/server.rb:23:in `start'
    /Users/keone/.rvm/rubies/ruby-1.8.7-p299/lib/ruby/1.8/webrick/server.rb:82:in `start'

Rendered rescues/_trace (186.4ms)
Rendered rescues/_request_and_response (0.3ms)
Rendering rescues/layout (200)

Thanks.