Recently [we identified a regression in `test_tls_reject_certificate`]( https://github.com/dask/distributed/pull/4019#pullrequestreview-462034533 ) where the following `assert` no longer works correctly: https://github.com/dask/distributed/blob/44bf0b981039ef5e474fbc5ccc6cd5a98b42e5e4/distributed/comm/tests/test_comms.py#L649