Confused about why an Elixir Ecto validation test is not working

I have a user model that looks like this:

defmodule MyApp.User do
  schema "users" do
    field :name, :string
    field :email, :string

    field :password, :string, virtual: true
    field :password_confirmation, :string, virtual: true

    timestamps
  end

  @required_fields ~w(name email)
  @optional_fields ~w()

  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
    |> validate_length(:password, min: 8)
  end
end

And the test in question makes sure the password length validation works.

defmodule MyApp.UserTest do
  use MyApp.ModelCase

  alias MyApp.User
  alias MyApp.Repo

  @valid_attrs %{
    name: "Uli Kunkel",  
    email: "nihilist@example.com", 
  }

  @invalid_attrs %{}

  test "validates the password is the correct length" do
    attrs = @valid_attrs |> Map.put(:password, "1234567")
    changeset = %User{} |> User.changeset(attrs)
    assert {:error, changeset} = Repo.insert(changeset)
  end
end

Only the test fails, even though the password is a character too short. What am I missing?