doctest not working for nonmatching function clause.

doctest_test.ex

defmodule ListExtensions do
  @moduledoc false

  @doc """
  Traverses all of the elements in given list
  and then sums the result and returns it.

  Does not accept types other than list.

  ### Examples

  iex> ListExtensions.mapsum [1,2,3,4], &1 * &1
  1 + 4 + 9 + 16

  iex> ListExtensions.mapsum {1,2,3,4}, &1 * &1
  ** (FunctionClauseError)
  """
  def mapsum(list, fun)
  when is_list(list) do
    do_mapsum(list, 0, fun)
  end

  defp do_mapsum([], sum, _), do: sum

  defp do_mapsum([ head | list ], sum, fun) do
    do_mapsum(list, sum + fun.(head), fun) 
  end
end


ExUnit.start

defmodule ListExtensions.Test do
  use ExUnit.Case, async: true

  doctest ListExtensions
end