diff --git a/plugins/MakerbotWriter/MakerbotWriter.py b/plugins/MakerbotWriter/MakerbotWriter.py
index 521db04b5b..2a6243828c 100644
--- a/plugins/MakerbotWriter/MakerbotWriter.py
+++ b/plugins/MakerbotWriter/MakerbotWriter.py
@@ -154,7 +154,7 @@ class MakerbotWriter(MeshWriter):
             if bounds is None:
                 bounds = node_bounds
             else:
-                bounds += node_bounds
+                bounds = bounds + node_bounds
 
         if bounds is not None:
             meta["bounding_box"] = {