Why I'm getting nil on second argument of `Kernel.<>()`?

I’m trying to build a telegram bot with Nadia, and I thought that would be a good idea, to wrap some functionalities with macros, for studying purposes!

Here’re my macros:

#router.ex
defmodule Lovelace.Router
  [...]
  defp generate_command(command, handler) do
    quote do
      def do_match_message(
            %{
              message: %{
                text: "/" <> unquote(command)
              }
            } = var!(update)
          ) do
        handle_message(unquote(handler), [var!(update)])
      end

      def do_match_message(
            %{
              message: %{
                text: "/" <> unquote(command) <> " " <> _
              }
            } = var!(update)
          ) do
        handle_message(unquote(handler), [var!(update)])
      end

      def do_match_message(
            %{
              message: %{
                text: "/" <> unquote(command) <> "@" <> unquote(@bot_name)
              }
            } = var!(update)
          ) do
        handle_message(unquote(handler), [var!(update)])
      end

      def do_match_message(
            %{
              message: %{
                text: "/" <> unquote(command) <> "@" <> unquote(@bot_name) <> " " <> _
              }
            } = var!(update)
          ) do
        handle_message(unquote(handler), [var!(update)])
      end
    end
  end

    # Helpers

  def handle_message({module, function}, update)
      when is_atom(function) and is_list(update) do
    Task.start(fn ->
      apply(module, function, [hd(update)])
    end)
  end

  def handle_message({module, function}, update)
      when is_atom(function) do
    Task.start(fn ->
      apply(module, function, [update])
    end)
  end

  def handle_message(function, _update)
      when is_function(function) do
    Task.start(fn ->
      function.()
    end)
  end

  def handle_message(_, _), do: nil

  ## Command

  defmacro command(commands, do: function)
           when is_list(commands) do
    Enum.map(commands, fn command ->
      generate_command(command, function)
    end)
  end

  defmacro command(command, do: function) do
    generate_command(command, function)
  end

  defmacro command(commands, module, function)
           when is_list(commands) do
    Enum.map(commands, fn command ->
      generate_command(command, {module, function})
    end)
  end

  defmacro command(command, module, function) do
    generate_command(command, {module, function})
  end
  [...]
end

And I’m using as:

#commands.ex
defmodule Lovelace.Commands do
  @moduledoc """
  Define all bot's commands
  """

  use Lovelace.Router
  use Lovelace.Commander

  import Lovelace.Helpers

  alias Lovelace.Commands.Outside
 
  command "welcome" do
    Logger.log(:info, "Command /welcome")

    ~s(Seja bem vindo ao grupo de Ciência da Computação! )
    |> Kernel.<>(
      ~s|Leia as regras do grupo no pinado e visite nosso GitHub(https://github.com/cciuenf) |
    )
    |> Kernel.<>(~s(Digite /ajuda para ver mais comandos!))
    |> send_message()
  end
end

However, when compiling, I’m geeting:

Compiling 2 files (.ex)

== Compilation error in file lib/lovelace/commands.ex ==
** (ArgumentError) expected binary argument in <> operator but got: nil
    (elixir 1.11.3) lib/kernel.ex:1821: Kernel.wrap_concatenation/3
    (elixir 1.11.3) lib/kernel.ex:1812: Kernel.extract_concatenations/2
    (elixir 1.11.3) lib/kernel.ex:1808: Kernel.extract_concatenations/2
    (elixir 1.11.3) expanding macro: Kernel.<>/2
    lib/lovelace/commands.ex:13: Lovelace.Commands.do_match_message/1

The Error is thrown on

command "welcome"...

in the commands.ex file!

But I ain’t finding where is the error… Could you help me?