diff --git a/synapse/storage/__init__.py b/synapse/storage/__init__.py
index 672ed6971..46d420b63 100644
--- a/synapse/storage/__init__.py
+++ b/synapse/storage/__init__.py
@@ -206,7 +206,7 @@ class DataStore(RoomMemberStore, RoomStore,
         unrec = {
             k: v
             for k, v in event.get_full_dict().items()
-            if k not in vals.keys() and k is not "deleted"
+            if k not in vals.keys() and k not in ["deleted", "pruned"]
         }
         vals["unrecognized_keys"] = json.dumps(unrec)