diff --git a/synapse/api/events/__init__.py b/synapse/api/events/__init__.py
index c5a3334b9..f9653e0b2 100644
--- a/synapse/api/events/__init__.py
+++ b/synapse/api/events/__init__.py
@@ -41,6 +41,7 @@ class SynapseEvent(JsonEncodedObject):
         "room_id",
         "user_id",  # sender/initiator
         "content",  # HTTP body, JSON
+        "state_key",
     ]
 
     internal_keys = [