How do I use Ecto cast_embed correctly on this Map?

I’m trying to figure out how to use cast_embed but I don’t understand what I’m doing wrong.

defmodule Operation do
  use Ecto.Schema

  embedded_schema do
    field :amount, :integer
    field :type, :string
    embeds_one :item, Item
  end
end

defmodule Item do
  use Ecto.Schema

  embedded_schema do
    field :item_code, :string
    field :item_name, :string
  end
end

with the following data

{ 
  "item": {
    "item_code": "A1",
    "item_name": "NameA1"
  },
  "type": "type",
  "amount": 4
}

d = %{
  "amount" => 4,
  "item" => %{"item_code" => "A1", "item_name" => "NameA1"},
  "type" => "type"
}

cast(%Operation{}, d, [:amount, :type]) |> validate_required([:amount, :type]) |> cast_embed(:item)

This throws an error:

** (UndefinedFunctionError) function Item.changeset/2 is undefined or private. Did you mean one of:

  * __changeset__/0

Item.changeset(%Item{id: nil, item_code: nil, item_name: nil}, %{"item_code" => "A1", "item_name" => "NameA1"})
(ecto) lib/ecto/changeset.ex:736: anonymous fn/4 in Ecto.Changeset.on_cast_default/2
(ecto) lib/ecto/changeset/relation.ex:111: Ecto.Changeset.Relation.do_cast/5
(ecto) lib/ecto/changeset/relation.ex:292: Ecto.Changeset.Relation.single_change/5
(ecto) lib/ecto/changeset.ex:715: Ecto.Changeset.cast_relation/4

Been stuck on this for a while and I can’t find a working example of how to do this online, any ideas what I’m doing wrong here?