Schienen + CarrierWave: NoMethodError: undefined method `name' for nil:NilClass

Ich bin mit CarrierWave mit Rails 3.1. Ich erhalte die folgende Fehlermeldung, wenn ich das Formular abschicken (der Versuch ein Bild hochzuladen):

Fehlermeldung:

ActiveRecord::StatementInvalid in Admin::PostsController#create

NoMethodError: undefined method `name' for nil:NilClass: INSERT INTO "posts" ("body", "created_at", "draft", "image", "post_type", "title", "updated_at", "user_id") VALUES (?, ?, ?, ?, ?, ?, ?, ?)
Rails.root: /Users/aziz/Sandbox/ruby/rails/Tumblelog

Application Trace | Framework Trace | Full Trace
app/controllers/admin/posts_controller.rb:18:in `create'
Request

Parameters:

{"utf8"=>"✓",
 "authenticity_token"=>"za+zNRDGNCcujnCmO726cWCo2ze1rgaXv5bL17JGaek=",
 "post"=>{"image"=>#<ActionDispatch::Http::UploadedFile:0x000001014aeff0 @original_filename="AzizLight.jpeg",
 @content_type="image/jpeg",
 @headers="Content-Disposition: form-data; name=\"post[image]\"; filename=\"AzizLight.jpeg\"\r\nContent-Type: image/jpeg\r\n",
 @tempfile=#<File:/var/folders/ky/2ddtbt0d7k1g__2ctr8njcfc0000gn/T/RackMultipart20110918-21704-hp2ajt>>,
 "draft"=>"0",
 "user_id"=>"2",
 "post_type"=>"image"},
 "commit"=>"Post"}

Das problem ist, dass ich nicht weiß, wo diese name kommt und ich weiß nicht, was variable wird null, so kann ich nicht richtig Debuggen (ich habe versucht, zu Debuggen ein Protokoll vor der Frage hier).
Zeile 18 entspricht der @post.save Zeile in der folgenden controller:

PostsController:

# ...

def new
  @post = Post.new
  @form_html_options = (params[:post_type] == "image") ? { :multipart => true } : {}
  @form_partial = get_form_partial(params[:post_type])
  redirect_to admin_posts_path, :alert => "You tried to create an unknown type of post..." if @form_partial.nil?
  @title = "Creating a new post..."
end

def create
  @post = Post.new(params[:post])
  if @post.save
    flash[:success] = "Post created successfully!"
    redirect_to admin_post_path(@post)
  else
    @title = "Creating a new post..."
    @form_partial = get_form_partial(params[:post][:post_type])
    render 'new'
  end
end

# ...

Hier andere Dateien, die benötigt werden könnten, um vor Ort das problem:

Post (Modell):

attr_accessible :title, :body, :user_id, :draft, :post_type, :image

belongs_to :user

mount_uploader :image_url, ImageUploader

ImageUploader:

class ImageUploader < CarrierWave::Uploader::Base
  include CarrierWave::RMagick

  storage :fog

  def extension_white_list
    %w(jpg jpeg gif png)
  end
end

neu.html-Code.erb:

<h1><%= @title %></h1>

<%= form_for @post, :url => admin_posts_path, :html => @form_html_options do |f| %>
  <%= render 'form', :f => f %>
<% end %>

_form.html-Code.erb:

<%= render 'error_messages' %>

<%= render @form_partial, :f => f %>

<p class="drop-down">
  <%= f.label :draft, 'Status' %>
  <%= f.select(:draft, options_for_select([["Published", 0], ["Draft", 1]], (@post.new_record? ? 0: @post.draft))) %>
</p>

<%= f.hidden_field :user_id, :value => @post.user_id || current_user.id %>
<%= f.hidden_field :post_type, :value => @post.post_type || params[:post_type] %>

<p class="button"><%= f.submit "Post", :disable_with => 'Posting...' %></p>

_image_form.html-Code.erb (@form_partial):

<p><%= f.file_field :image %></p>

Also was wirklich Los ist, bitte?