Rails 6 Parallel Tests fail with improbable ArgumentError

When running tests on our newly upgraded Rails 6.0.2.1, and am trying the new parallel testing feature.

When run with parallelism of anything but 1, I get: ArgumentError: wrong number of arguments (given 61, expected 1)

Tests pass when run with PARALLEL_WORKERS=1, so I believe my test suite is ok, and this is an issue with the runner, or my configuration of it.

Every single test has the same error, below is one example:

Error:
NotificationChannels::AmazonSQSTest#test_send_open_notification:
ArgumentError: wrong number of arguments (given 61, expected 1)

rails test test/models/notification_channels/amazon_sqs_test.rb:32

Line 32 is the test definition itself, nothing unusual: test "send open notification" do. We also have tests in the form of def test_foo that are failing in the same manner.

The various numbered test databases appear to have been created and populated as I'd expect.

I'm at a loss at what to look at.

Versions of stuff:

• Rails 6.0.2.1
• minitest 5.10.3
• Ruby 2.6.5