diff --git a/changelog.d/6263.misc b/changelog.d/6263.misc
new file mode 100644
index 0000000000..7b1bb4b679
--- /dev/null
+++ b/changelog.d/6263.misc
@@ -0,0 +1 @@
+Change cache descriptors to always return deferreds.
diff --git a/synapse/push/bulk_push_rule_evaluator.py b/synapse/push/bulk_push_rule_evaluator.py
index 22491f3700..2bbdd11941 100644
--- a/synapse/push/bulk_push_rule_evaluator.py
+++ b/synapse/push/bulk_push_rule_evaluator.py
@@ -79,7 +79,7 @@ class BulkPushRuleEvaluator(object):
             dict of user_id -> push_rules
         """
         room_id = event.room_id
-        rules_for_room = self._get_rules_for_room(room_id)
+        rules_for_room = yield self._get_rules_for_room(room_id)
 
         rules_by_user = yield rules_for_room.get_rules(event, context)
 
diff --git a/synapse/storage/data_stores/main/roommember.py b/synapse/storage/data_stores/main/roommember.py
index e47ab604dd..bc04bfd7d4 100644
--- a/synapse/storage/data_stores/main/roommember.py
+++ b/synapse/storage/data_stores/main/roommember.py
@@ -720,7 +720,7 @@ class RoomMemberWorkerStore(EventsWorkerStore):
         # See bulk_get_push_rules_for_room for how we work around this.
         assert state_group is not None
 
-        cache = self._get_joined_hosts_cache(room_id)
+        cache = yield self._get_joined_hosts_cache(room_id)
         joined_hosts = yield cache.get_destinations(state_entry)
 
         return joined_hosts
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index 5ac2530a6a..0e8da27f53 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -438,7 +438,7 @@ class CacheDescriptor(_CacheDescriptorBase):
                 if isinstance(cached_result_d, ObservableDeferred):
                     observer = cached_result_d.observe()
                 else:
-                    observer = cached_result_d
+                    observer = defer.succeed(cached_result_d)
 
             except KeyError:
                 ret = defer.maybeDeferred(
@@ -482,9 +482,8 @@ class CacheListDescriptor(_CacheDescriptorBase):
     Given a list of keys it looks in the cache to find any hits, then passes
     the list of missing keys to the wrapped function.
 
-    Once wrapped, the function returns either a Deferred which resolves to
-    the list of results, or (if all results were cached), just the list of
-    results.
+    Once wrapped, the function returns a Deferred which resolves to the list
+    of results.
     """
 
     def __init__(
@@ -618,7 +617,7 @@ class CacheListDescriptor(_CacheDescriptorBase):
                 )
                 return make_deferred_yieldable(d)
             else:
-                return results
+                return defer.succeed(results)
 
         obj.__dict__[self.orig.__name__] = wrapped
 
diff --git a/tests/util/caches/test_descriptors.py b/tests/util/caches/test_descriptors.py
index 5713870f48..f907903511 100644
--- a/tests/util/caches/test_descriptors.py
+++ b/tests/util/caches/test_descriptors.py
@@ -325,9 +325,9 @@ class DescriptorTestCase(unittest.TestCase):
         self.assertEqual(len(obj.fn.cache.cache), 3)
 
         r = obj.fn(1, 2)
-        self.assertEqual(r, ["spam", "eggs"])
+        self.assertEqual(r.result, ["spam", "eggs"])
         r = obj.fn(1, 3)
-        self.assertEqual(r, ["chips"])
+        self.assertEqual(r.result, ["chips"])
         obj.mock.assert_not_called()
 
     def test_cache_iterable_with_sync_exception(self):