Elixir characters Range

I play with Range in Elixir.
It works fine:

>1..10 |> Enum.map fn(x) -> x |> IO.puts end
>1
>..
>10
>[:ok, :ok, :ok, :ok, :ok, :ok, :ok, :ok, :ok, :ok]

But here have error:

>'a'..'z' |> Enum.map fn(x) -> x |> IO.puts end
 ** (Protocol.UndefinedError) protocol Range.Iterator not implemented for 'a'
 /Users/elix_lang/lib/elixir/lib/range.ex:7: Range.Iterator.impl_for!/1

I come from Ruby familiar with Erlang
I want to understand what's the problem?
Thanks!!!