** (Mix) The task "coveralls" could not be found

I’m trying to install and run excoveralls in an umbrella app. Installation and compilation work just fine – I see the excoveralls folder inside dep and "0.16.0" inside the mix.lock. mix.exs includes:

{:excoveralls, "~> 0.16.0", only: [:dev], runtime: false},

However, when I try to run mix coveralls, I get

** (Mix) The task "coveralls" could not be found

Curiously, there is NOT a folder created inside of _build… which would explain why the mix task could not be found. But why didn’t this compile?

This is an umbrella app… the dependency was listed in the root mix.exs with the following modifier to the mix.exs:

test_coverage: [
        tool: ExCoveralls
      ],
      preferred_cli_env: [
        coveralls: :test,
        "coveralls.detail": :test
      ]

And I followed the documentation’s advice for umbrellas and added

test_coverage: [
        tool: ExCoveralls
      ],

to the other mix.exs files.

But still no dice.

The only thing that is unusual about this repository is that up till now, the built-in Erlang :cover library was being tapped to do coverage reports. We’d do this:

mix test --cover

So the only guess I have is that it’s somehow interfering with ExCoveralls, but I can’t pinpoint it. I have tried deleting the _build and deps directories, but the result is the same.

Anyone have ideas? Thanks in advance!