`mix test --cover` not writing cover data

[gilbertbw]

I have added excoveralls to my project, and set it up as so:

defmodule Blah.MixProject do
 def project do
    [
#...
      test_coverage: [tool: ExCoveralls],
      preferred_cli_env: [coveralls: :test]
    ]
  end
#...
  defp deps do
    [
# ...
      {:excoveralls, "~> 0.18.3"}
    ]
  end
end

In the readme it says

excoveralls/README.md

Line 520 in 610c538

Coverage data is generated when running `mix test --cover`, optionally with the `--export-coverage` flag to specify an output name.

But now when I run mix test --cover nothing gets written. Before I installed excoveralls mix test --cover did write out the built in coverage report.