undefined method `synchronize' for nil:NilClass

I just created a new app from scratch on rails4.0.0.rc1 with 1.9.3-p374 (which should work according to the guides).

However, I fail doing a simple create-action.

class Books
  def create
    @book = Book.new book_params

    if @book.save
      redirect_to @book
    else
      render action: :new
    end
  end

  private

  def book_params
    params.require(:book).permit(:name, :description)
  end
end

Doing a create here results a

undefined method `synchronize' for nil:NilClass

with the error pointing to the line of if @book.save.

Is that because of 1.9.3 or did I miss something?