mix test.coverage fails with an exception raised

/app # mix test.coverage

** (exit) an exception was raised:
    ** (MatchError) no match of right hand side value: :error
        (tools 3.5.3) cover.erl:1782: :cover.do_compile_beam2/6
        (tools 3.5.3) cover.erl:1672: :cover.do_compile_beam/3
        (stdlib 4.1) lists.erl:1320: :lists.map_1/2
        (stdlib 4.1) lists.erl:1315: :lists.map/2
        (tools 3.5.3) cover.erl:3150: anonymous fn/2 in :cover.pmap_spawn/4
    (tools 3.5.3) cover.erl:777: :cover.call/1
    (mix 1.14.2) lib/mix/tasks/test.coverage.ex:216: anonymous fn/2 in Mix.Tasks.Test.Coverage.cover_compile/1
    (elixir 1.14.2) lib/enum.ex:2468: Enum."-reduce/3-lists^foldl/2-0-"/3
    (mix 1.14.2) lib/mix/tasks/test.coverage.ex:215: Mix.Tasks.Test.Coverage.cover_compile/1
    (mix 1.14.2) lib/mix/tasks/test.coverage.ex:148: Mix.Tasks.Test.Coverage.run/1
    (mix 1.14.2) lib/mix/task.ex:421: anonymous fn/3 in Mix.Task.run_task/4
    (mix 1.14.2) lib/mix/cli.ex:84: Mix.CLI.run_task/2

We can start with you showing us your coverage config in mix.exs?

I am just starting out with elixir and phoenix FYI. AFAIK I do not have any test coverage configuration in mix.exs. I did not believe I need any???

defmodule Sct.MixProject do
  @moduledoc false

  use Mix.Project

  def project do
      app: :sct,
      version: "0.2.0",
      elixir: "~> 1.14",
      elixirc_paths: elixirc_paths(Mix.env()),
      start_permanent: Mix.env() == :prod,
      aliases: aliases(),
      deps: deps(),
      dialyzer: [
        plt_file: {:no_warn, "priv/plts/dialyzer.plt"}

Ah, you’re using the one that’s built in. Not well-versed in that, apologies.

I and others usually use excoveralls.

Sorry - I was under the impression that excoveralls did the reporting - not the analysis that generated the coverage.

Now you confused me more. :003:

Are you using ex_coveralls or not?

If you do, it needs some configuration keys in mix.exs, detailed in its docs on HexDocs.

I am NOT using ex_coveralls.

I used mix test.coverage to generate the error trace above on elixir 1.14.2 and phoenix 1.7-rc0 and erlang 25.1.