diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index 4f4914467c..4ad37188ba 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -98,7 +98,7 @@ def serialize_event(hs, e):
     d = {k: v for k, v in e.get_dict().items()}
     if "age_ts" in d["unsigned"]:
         now = int(hs.get_clock().time_msec())
-        d["unsigned"]["age"] = now - d["unsigned"]["age_ts"]
+        d["age"] = now - d["unsigned"]["age_ts"]
         del d["unsigned"]["age_ts"]
 
     d["user_id"] = d.pop("sender", None)