diff --git a/Tools/PyUtils/python/MetaDiff.py b/Tools/PyUtils/python/MetaDiff.py
index 6df41e7eee264026d7edabaae4f6f1fc54f378fe..7c8224d972ebfed17408f92496d59e3e96e3efd5 100644
--- a/Tools/PyUtils/python/MetaDiff.py
+++ b/Tools/PyUtils/python/MetaDiff.py
@@ -186,7 +186,7 @@ def compare(obj1, obj2, parent_key=None, ordered=False, diff_format="simple"):
                     print_diff_dict_keys(parent_key, obj1, obj2, diff_format)
                 ]
             else:
-                for key in sorted(set(obj1.keys() + obj2.keys())):
+                for key in sorted(set(obj1.keys()) | set(obj2.keys())):
                     if parent_key:
                         child_key = "{}/{}".format(parent_key, key)
                     else: