Insert multiple rows at once with Ecto. "protocol Enumerable not implemented for #Ecto.Changeset"

I have two tables. A table of topics which has_many tweets. My table of tweets belongs_to a topic.

Topic Schema:

defmodule Sentiment.Topic do
  use Sentiment.Web, :model

  schema "topics" do
    field :title, :string

    has_many :tweets, Sentiment.Tweet
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:title])
    |> validate_required([:title])
  end
end

Tweet Schema:

defmodule Sentiment.Tweet do
  use Sentiment.Web, :model

  schema "tweets" do
    field :message, :string
    belongs_to :topic, Sentiment.Topic

  end

  @doc """
  Builds a changeset based on the `struct` and `params`.
  """
  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:message])
    |> validate_required([:message])
  end
end

I am attempting to insert a topic into my table, followed by 500 tweets after I run a twitter search for that topic.

In my controller, I use Ecto.Multi to group my repo operations, however, each time I run my operation I get an error protocol Enumerable not implemented for #Ecto.Changeset<action: nil, changes: %{message: "\"aloh....

This is how I am attempting to insert my topic first, obtain it's id, and then insert a tweet message with the associated id with one transaction.

 def create(conn, %{"topic" => topic}) do
    # create a topic changeset
    topic_changeset = Topic.changeset(%Topic{}, topic)

    # obtain a list of tweet messages: ["hello", "a tweet", "sup!"]
    %{"title" => title} = topic
    all_tweets = title
    |> Twitter.search

# create an Ecto.Multi struct.
multi =
  Ecto.Multi.new
  |> Ecto.Multi.insert(:topics, topic_changeset) #insert topic
  |> Ecto.Multi.run(:tweets, fn %{topics: topic} ->
    changeset_tweets = all_tweets
    |> Enum.map(fn(tweet) ->
      %{topic_id: topic.id, message: tweet}
    end)

    Repo.insert_all(Tweet, changeset_tweets)

  end)

      # Run the transaction
      case Repo.transaction(multi) do # ERROR HERE!
        {:ok, result} ->
          conn
          |> put_flash(:info, "Success!")
          |> redirect(to: topic_path(conn, :index))
        {:error, :topics, topic_changeset, %{}} ->
          conn
          |> put_flash(:error, "Uh oh...")
          |> render("new.html", changeset: topic_changeset)
        {:error, :tweets, topic_changeset, %{}} ->
          conn
          |> put_flash(:error, "Something really bad happened...")
          |>render("new.html", changeset: topic_changeset)
      end
  end

How can I insert_all about 500 rows in one transaction using Ecto.Multi?

Update I have converted the list of changesets into a list of maps and my error has changed to something even more confusing.