diff --git a/src/blenderbim/blenderbim/bim/module/system/data.py b/src/blenderbim/blenderbim/bim/module/system/data.py
index abeddb605b..645c770440 100644
--- a/src/blenderbim/blenderbim/bim/module/system/data.py
+++ b/src/blenderbim/blenderbim/bim/module/system/data.py
@@ -210,7 +210,7 @@ class SystemDecorationData:
             decorated_elements = set(ifcopenshell.util.system.get_system_elements(active_system))
 
         if not decorated_elements:
-            decorated_elements += ObjectSystemData.data["connected_elements"]
+            decorated_elements.update(ObjectSystemData.data["connected_elements"])
 
         return decorated_elements