AttributeError: assert_message_contains

[pulltab]

I am new to using distest.

I am using the current main branch (9fcd640) against discord.py==1.7.2.

I am attempting to test a simple flow:

• Tester posts a message in the test channel.
• Target responds with a DM to Tester
• Assert some details about the response

When calling assert_message_contains, I get the following error:

Traceback (most recent call last):
  File "/Users/john.koenig/git/MatchmakingBot/.venv/lib/python3.9/site-packages/discord/client.py", line 343, in _run_event
    await coro(*args, **kwargs)
  File "/Users/john.koenig/git/MatchmakingBot/.venv/lib/python3.9/site-packages/distest/bot.py", line 250, in on_ready
    await self.run_tests(self._channel, self._test_to_run)
  File "/Users/john.koenig/git/MatchmakingBot/.venv/lib/python3.9/site-packages/distest/bot.py", line 194, in run_tests
    await self._run_by_predicate(channel)
  File "/Users/john.koenig/git/MatchmakingBot/.venv/lib/python3.9/site-packages/distest/bot.py", line 117, in _run_by_predicate
    await self.run_test(test, channel, stop_error=True)
  File "/Users/john.koenig/git/MatchmakingBot/.venv/lib/python3.9/site-packages/distest/bot.py", line 81, in run_test
    await test.func(test_interface)
  File "/Users/john.koenig/git/MatchmakingBot/tester.py", line 21, in test_ping
    await interface.assert_message_contains("&lfg mid", "pong!")
  File "/Users/john.koenig/git/MatchmakingBot/.venv/lib/python3.9/site-packages/distest/TestInterface/_message.py", line 28, in assert_message_contains
    if substring not in message.content:
AttributeError: 'str' object has no attribute 'content'

Am I doing something incorrectly? Is there a better way to wait for DM contents?