Mongoid: Save an array of strings throws TypeError?

Question

What am I doing wrong? I have this Mongoid-based model:

class ReadableThing
  include Mongoid::Document
  field :shop,    type: Integer
  field :user,                 type: Integer
  field :title,                type: String
  field :s_title,              type: String
  field :s_desc,               type: String
  field :is_part,              type: Boolean
  field :pieces,               type: Array

  def self.with_shop_only(shop, with_parts)
    ReadableThing.where(shop_id: shop, user: nil, is_part: with_parts)
  end

  def self.with_shop_and_user(shop, user_id, with_parts)
    ReadableThing.where(brokerage_shop_id: shop, user: user_id, is_part: with_parts)
  end
end

and when running this code:

ReadableThing.create({
  shop: 2,
  user: nil,
  title: "Ooohh samples",
  s_title: "SPL",
  s_desc: nil,
  is_part: true,
  pieces: ["this","is","a","sample"],
})

I get this error:

TypeError: no implicit conversion of String into Integer

I've been able to track the thing down to the "pieces" array, everything works just perfect without it. I can't, though, figure out what I'm doing wrong.

Can anyone please lend me a hand here?