diff --git a/synapse/appservice/__init__.py b/synapse/appservice/__init__.py
index 2255ea23e987..c5e9397b4562 100644
--- a/synapse/appservice/__init__.py
+++ b/synapse/appservice/__init__.py
@@ -259,8 +259,8 @@ async def is_interested_in_room(
     async def is_interested_in_event(
         self,
         event: EventBase,
+        store: "DataStore",
         cache_context: _CacheContext,
-        store: Optional["DataStore"] = None,
     ) -> bool:
         """Check if this service is interested in this event.
 
@@ -288,10 +288,7 @@ async def is_interested_in_event(
         ):
             return True
 
-        # TODO: The store is only optional here to aid testing this function. We should
-        #  instead convert the tests to use HomeServerTestCase in order to get a working
-        #  database instance.
-        if store is not None and await self.is_interested_in_room(
+        if await self.is_interested_in_room(
             event.room_id, store, on_invalidate=cache_context.invalidate
         ):
             return True
diff --git a/synapse/handlers/appservice.py b/synapse/handlers/appservice.py
index 61adafb0ebed..03fc73407f19 100644
--- a/synapse/handlers/appservice.py
+++ b/synapse/handlers/appservice.py
@@ -769,7 +769,7 @@ async def _get_services_for_event(
         # inside of a list comprehension anymore.
         interested_list = []
         for s in services:
-            if await s.is_interested_in_event(event, store=self.store):
+            if await s.is_interested_in_event(event, self.store):
                 interested_list.append(s)
 
         return interested_list
diff --git a/tests/appservice/test_appservice.py b/tests/appservice/test_appservice.py
index 38bbd8d11469..fb492216443b 100644
--- a/tests/appservice/test_appservice.py
+++ b/tests/appservice/test_appservice.py
@@ -19,6 +19,7 @@
 from synapse.appservice import ApplicationService, Namespace
 
 from tests import unittest
+from tests.test_utils import simple_async_mock
 
 
 def _regex(regex: str, exclusive: bool = True) -> Namespace:
@@ -39,6 +40,8 @@ def setUp(self):
         )
 
         self.store = Mock()
+        self.store.get_aliases_for_room = simple_async_mock(return_value=[])
+        self.store.get_users_in_room = simple_async_mock(return_value=[])
 
     @defer.inlineCallbacks
     def test_regex_user_id_prefix_match(self):
@@ -47,7 +50,7 @@ def test_regex_user_id_prefix_match(self):
         self.assertTrue(
             (
                 yield defer.ensureDeferred(
-                    self.service.is_interested_in_event(self.event)
+                    self.service.is_interested_in_event(self.event, self.store)
                 )
             )
         )
@@ -59,7 +62,7 @@ def test_regex_user_id_prefix_no_match(self):
         self.assertFalse(
             (
                 yield defer.ensureDeferred(
-                    self.service.is_interested_in_event(self.event)
+                    self.service.is_interested_in_event(self.event, self.store)
                 )
             )
         )
@@ -73,7 +76,7 @@ def test_regex_room_member_is_checked(self):
         self.assertTrue(
             (
                 yield defer.ensureDeferred(
-                    self.service.is_interested_in_event(self.event)
+                    self.service.is_interested_in_event(self.event, self.store)
                 )
             )
         )
@@ -102,7 +105,7 @@ def test_regex_room_id_no_match(self):
         self.assertFalse(
             (
                 yield defer.ensureDeferred(
-                    self.service.is_interested_in_event(self.event)
+                    self.service.is_interested_in_event(self.event, self.store)
                 )
             )
         )
@@ -207,7 +210,7 @@ def test_interested_in_self(self):
         self.assertTrue(
             (
                 yield defer.ensureDeferred(
-                    self.service.is_interested_in_event(self.event)
+                    self.service.is_interested_in_event(self.event, self.store)
                 )
             )
         )