Can't use Stream.transform with a map accumulator?

transform.exs

iex(1)> list = [1,2,3]
[1,2,3]
iex(2)> Stream.transform(list, fn -> %{} end, fn (x, a) -> Map.put(a, x, "#{x}") end, fn (map) -> Map.values(map) end) |> Enum.to_list
** (TryClauseError) no try clause matching: %{1 => "1"}
    (elixir) lib/stream.ex:739: Stream.do_transform/8
    (elixir) lib/enum.ex:1636: Enum.reduce/3
    (elixir) lib/enum.ex:2346: Enum.to_list/1