diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index b1ffe73229bb..7a7b0377717e 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -554,11 +554,15 @@ async def encode_room(
 
             if room.preview:
                 if "event" in room.preview:
-                    room.preview["event"] = self._event_serializer.serialize_events(
+                    room.preview[
+                        "event"
+                    ] = await self._event_serializer.serialize_events(
                         [room.preview["event"]],
                         time_now,
                         config=serialize_options,
-                    )[0]
+                    )[
+                        0
+                    ]
                 result["com.beeper.inbox.preview"] = room.preview
 
         return result