ExUnit.DocTest: Doesn't run test when "iex>" prefix contains numbers

gistfile1.txt

# using Elixir 1.2.2

$ mix new my_app
$ cd my_app
$ vi lib/my_app.ex

defmodule MyApp do
  @doc """
  iex(1)> MyApp.foo
  123
  """
  def foo do
    456
  end
end

$ mix test # <- runs 1 test, 0 failures

$ vi lib/my_app.ex

defmodule MyApp do
  @doc """
  iex> MyApp.foo
  123
  """
  def foo do
    456
  end
end

$ mix test # <- runs 2 tests, 1 failure