diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 618cb53629..55d11122ba 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1093,16 +1093,17 @@ class FederationHandler(BaseHandler):
         )
 
         if event:
-            # FIXME: This is a temporary work around where we occasionally
-            # return events slightly differently than when they were
-            # originally signed
-            event.signatures.update(
-                compute_event_signature(
-                    event,
-                    self.hs.hostname,
-                    self.hs.config.signing_key[0]
+            if self.hs.is_mine_id(event.event_id):
+                # FIXME: This is a temporary work around where we occasionally
+                # return events slightly differently than when they were
+                # originally signed
+                event.signatures.update(
+                    compute_event_signature(
+                        event,
+                        self.hs.hostname,
+                        self.hs.config.signing_key[0]
+                    )
                 )
-            )
 
             if do_auth:
                 in_room = yield self.auth.check_host_in_room(