'没有使用双嵌套形式将字符串隐式转换为整数'

Question

这是我的表格：

<%= simple_form_for @item do |item_builder| %>
  <div class="well">
  <%= item_builder.input :name %>
  <%= item_builder.input :description, as: :text %>
  <%= item_builder.input :tag_list %>

  <%= item_builder.simple_fields_for :user_items do |user_item_builder| %>
    <%= user_item_builder.input :foo, as: :hidden, input_html: { value: "bar" } %>

    <%= user_item_builder.simple_fields_for :user_item_images do |user_item_images_builder| %>
      <%= user_item_images_builder.input :picture, as: :file, 
                input_html: { multiple: true,
                              name: "item[user_items_attributes][user_item_images_attributes][][picture]" } %>
      <% end %>
  <% end %>
  </div>
  <div class="clearfix">
    <%= item_builder.submit 'Submit new item request'%>
  </div>
<% end %>

和我的items_controller

  def new
    @item = Item.new
    @user_item = @item.user_items.build
    @user_item.user_item_images.build
  end

  def create
    @item = Item.new item_params
    @item.user_items.first.user_id = current_user.id
    if @item.save
      redirect_to items_path, notice: "Thank you for your item request!"
    else
      render :new
    end
  end

  def item_params
    params.require(:item).permit(:name, :description, :tag_list,
        user_items_attributes: [:item_id, 
        user_item_images_attributes: [:user_item_id,   :picture]]).merge(created_by: current_user.id, status: Item::STATUS[:pending])
  end

我收到的错误no implicit conversion of String into Integer指向我create操作的第一行。 Web控制台中未定义item_params。我的错误可能是什么想法？