cannot add sync case to test suite after the suite starts running

[woylie]

When I try running mix exavier.test, it terminates with an error like this (with a different module named each time):

[error] Task #PID<0.6433.0> started from Exavier.Server terminating
** (RuntimeError) cannot add sync case named MyApp.SomeTest to test suite after the suite starts running
    (ex_unit 1.12.1) lib/ex_unit/server.ex:21: ExUnit.Server.add/2
    (elixir 1.12.1) src/elixir_module.erl:369: anonymous fn/5 in :elixir_module.expand_callback/6
    (elixir 1.12.1) src/elixir_module.erl:368: :elixir_module.expand_callback/6
    (stdlib 3.14.2.1) lists.erl:1267: :lists.foldl/3
    (elixir 1.12.1) src/elixir_module.erl:150: :elixir_module.compile/5
    (stdlib 3.14.2.1) erl_eval.erl:680: :erl_eval.do_apply/6
    (elixir 1.12.1) src/elixir.erl:280: :elixir.recur_eval/3
    (elixir 1.12.1) src/elixir.erl:265: :elixir.eval_forms/3
    (elixir 1.12.1) src/elixir_compiler.erl:46: :elixir_compiler.eval_forms/3
    (elixir 1.12.1) src/elixir_lexical.erl:15: :elixir_lexical.run/3
    (elixir 1.12.1) src/elixir_compiler.erl:18: :elixir_compiler.quoted/3
    (elixir 1.12.1) lib/code.ex:1261: Code.require_file/2
    (exavier 0.3.0) lib/exavier/server.ex:49: anonymous fn/5 in Exavier.Server.handle_call/3
    (elixir 1.12.1) lib/enum.ex:930: Enum."-each/2-lists^foreach/1-0-"/2
    (elixir 1.12.1) lib/task/supervised.ex:90: Task.Supervised.invoke_mfa/2
    (elixir 1.12.1) lib/task/supervised.ex:35: Task.Supervised.reply/5
    (stdlib 3.14.2.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
Function: &:erlang.apply/2
    Args: [#Function<2.82788263/1 in Exavier.Server.handle_call/3>, [{"test/my_app/some_test.exs", %{file: '/Users/whoever/my_app/apps/my_app/lib/my_app/some.ex', lines_to_mutate: [16, 22, 25, 26], module: MyApp.Some}}]]

Exavier 0.3.0
Elixir 1.12.1
Erlang/OTP 23