Dialyzer seems not to expect System.cmd to return a non-zero code

I ran dialyzer on this new code (with both Elixir 1.7 and 1.12) and am getting the error:

The pattern
variable_error

can never match, because previous clauses completely cover the type
:ok.

It points at the line in maybe_gen_certificates that does the put_error. I don’t understand why. The spec for generate_certificates should be correct, specifying :ok | {atom(), atom()}, and the with statement should be able to fail with {_, non_zero_integer} and return that as response. What am I missing?

  defp maybe_gen_certificates(config, false) do
    case generate_certificates() do
      :ok -> config
      error -> put_error(config, :certificate, error)
    end
  end

  @spec generate_certificates() :: :ok | {atom(), atom()}
  def generate_certificates do
    {key, cert} = keys()

    write_keys_permissions({key, cert})

    response =
      with {_, 0} <- System.cmd("openssl", ~w(genrsa -out #{key} 2048)),
           {_, 0} <- System.cmd("openssl", ~w(rsa -in #{key} -out #{cert} -outform PEM -pubout)),
           do: :ok

    reset_keys_permissions({key, cert})
    response
  end

Thanks!