diff --git a/mypy.ini b/mypy.ini
index 7760ea3d42..78ecf49379 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -99,6 +99,9 @@ disallow_untyped_defs = True
 [mypy-synapse.logging.formatter]
 disallow_untyped_defs = True
 
+[mypy-synapse.logging.handlers]
+disallow_untyped_defs = True
+
 [mypy-synapse.rest.*]
 disallow_untyped_defs = True
 
diff --git a/synapse/logging/handlers.py b/synapse/logging/handlers.py
index af5fc407a8..fbd6ba8102 100644
--- a/synapse/logging/handlers.py
+++ b/synapse/logging/handlers.py
@@ -49,7 +49,7 @@ class PeriodicallyFlushingMemoryHandler(MemoryHandler):
         )
         self._flushing_thread.start()
 
-        def on_reactor_running():
+        def on_reactor_running() -> None:
             self._reactor_started = True
 
         reactor_to_use: IReactorCore
@@ -74,7 +74,7 @@ class PeriodicallyFlushingMemoryHandler(MemoryHandler):
         else:
             return True
 
-    def _flush_periodically(self):
+    def _flush_periodically(self) -> None:
         """
         Whilst this handler is active, flush the handler periodically.
         """