diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py

index 6fb8332f93..a306631094 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -243,7 +243,9 @@ class SyncHandler:
         self.presence_handler = hs.get_presence_handler()
         self.event_sources = hs.get_event_sources()
         self.clock = hs.get_clock()
-        self.response_cache = ResponseCache(hs, "sync")
+        self.response_cache = ResponseCache(
+            hs, "sync"
+        )  # type: ResponseCache[Tuple[Any, ...]]
         self.state = hs.get_state_handler()
         self.auth = hs.get_auth()
         self.storage = hs.get_storage()