ServerError in PRAW tests

test_modmail.py

from praw.models import ModmailMessage
import mock

from ... import IntegrationTest


class TestModmail(IntegrationTest):
    @mock.patch('time.sleep', return_value=None)
    def test_call__conversation_id(self, _):
        conversation_id = 'ik72'
        with self.recorder.use_cassette(
                'TestModmail.test_call__conversation_id'):
            conversation = self.reddit.subreddit('all').modmail(
                conversation_id)
            for message in conversation.messages:
                assert isinstance(message, ModmailMessage)

working.py

import praw

user_agent = "praw test"
r = praw.Reddit(user_agent=user_agent)


def test_call__conversation_id():
    conversation_id = 'ik72'
    conversation = r.subreddit('all').modmail(conversation_id)
    for message in conversation.messages:
        assert isinstance(message, praw.models.ModmailMessage)
        print(message)