1 files changed, 3 insertions, 1 deletions
diff --git a/synapse/util/caches/deferred_cache.py b/synapse/util/caches/deferred_cache.py
index 1adc92eb90..dd392cf694 100644
--- a/synapse/util/caches/deferred_cache.py
+++ b/synapse/util/caches/deferred_cache.py
@@ -283,7 +283,9 @@ class DeferredCache(Generic[KT, VT]):
         # we return a new Deferred which will be called before any subsequent observers.
         return observable.observe()
 
-    def prefill(self, key: KT, value: VT, callback: Callable[[], None] = None):
+    def prefill(
+        self, key: KT, value: VT, callback: Optional[Callable[[], None]] = None
+    ):
         callbacks = [callback] if callback else []
         self.cache.set(key, value, callbacks=callbacks)