diff --git a/synapse/handlers/presence.py b/synapse/handlers/presence.py
index 4e71e10295..e85c49742d 100644
--- a/synapse/handlers/presence.py
+++ b/synapse/handlers/presence.py
@@ -939,8 +939,7 @@ class PresenceHandler(object):
 
         @defer.inlineCallbacks
         def _process_presence():
-            if self._event_processing:
-                return
+            assert not self._event_processing
 
             self._event_processing = True
             try: