slug contains a nil value

I followed this episode on elixircasts page. I have problem with creating new records. When I try to add a new book I face such an error

cannot convert Bookstore.Inventory.Book to param, key :slug contains a nil value

I don't know where is the problem with my code. I have written the same code to create slugs for my authors' pages and it works. I would be grateful for pointing where I made a mistake. I am using {:slugify, "~> 1.3"}.

inventory/book.ex

      @derive {Phoenix.Param, key: :slug}
  schema "books" do
    field(:description, :string)
    field(:image_url, BookstoreWeb.Uploaders.ImageUploader.Type)
    field(:isbn, :string)
    field(:original_price, :float)
    field(:title, :string)
    field(:slug, :string)
    field(:year_published, :integer)

    belongs_to(:category, Bookstore.Genres.Category)
    belongs_to(:author, Bookstore.Writers.Author)

    timestamps()
  end

  @doc false
  def changeset(book, attrs) do
    book
    |> cast(attrs, [
      :author_id,
      :category_id,
      :isbn,
      :title,
      :description,
      :year_published,
      :original_price,
      :slug
    ])
    |> validate_required([:isbn, :title, :description, :year_published, :original_price])
    |> build_slug()
    |> cast_attachments(attrs, [:image_url])
  end

  def build_slug(changeset) do
    id = get_field(changeset, :id)
    title = get_field(changeset, :title)

    if id && title do
      name = Enum.join([id, title], "")
      slug = Slug.slugify(name)
      put_change(changeset, :slug, slug)
    else
      changeset
    end
  end

inventory.ex

  def get_book!(slug),
    do:
      Repo.get_by!(Book, slug: slug)
      |> Repo.preload(:category)
      |> Repo.preload(:author)

  def create_book(attrs \\ %{}) do
    %Book{}
    |> Book.changeset(attrs)
    |> Repo.insert()
  end