diff --git a/tests/unittest.py b/tests/unittest.py

index c1cb5933fa..b21e7f1221 100644
--- a/tests/unittest.py
+++ b/tests/unittest.py
@@ -45,7 +45,7 @@ from typing_extensions import Concatenate, ParamSpec, Protocol
 from twisted.internet.defer import Deferred, ensureDeferred
 from twisted.python.failure import Failure
 from twisted.python.threadpool import ThreadPool
-from twisted.test.proto_helpers import MemoryReactor
+from twisted.test.proto_helpers import MemoryReactor, MemoryReactorClock
 from twisted.trial import unittest
 from twisted.web.resource import Resource
 from twisted.web.server import Request
@@ -82,7 +82,7 @@ from tests.server import (
 )
 from tests.test_utils import event_injection, setup_awaitable_errors
 from tests.test_utils.logging_setup import setup_logging
-from tests.utils import default_config, setupdb
+from tests.utils import checked_cast, default_config, setupdb
 
 setupdb()
 setup_logging()
@@ -296,7 +296,12 @@ class HomeserverTestCase(TestCase):
 
         from tests.rest.client.utils import RestHelper
 
-        self.helper = RestHelper(self.hs, self.site, getattr(self, "user_id", None))
+        self.helper = RestHelper(
+            self.hs,
+            checked_cast(MemoryReactorClock, self.hs.get_reactor()),
+            self.site,
+            getattr(self, "user_id", None),
+        )
 
         if hasattr(self, "user_id"):
             if self.hijack_auth: