diff --git a/app/serializers/user_serializer.rb b/app/serializers/user_serializer.rb
index cb183a2a7f0..ba43aaff5da 100644
--- a/app/serializers/user_serializer.rb
+++ b/app/serializers/user_serializer.rb
@@ -189,7 +189,7 @@ class UserSerializer < BasicUserSerializer
       }
     end
 
-    keys.sort! { |a, b| a.last_used_at.to_time > b.last_used_at.to_time }
+    keys.sort! { |a, b| a[:last_used_at].to_time > b[:last_used_at].to_time }
     keys.length > 0 ? keys : nil
   end