diff --git a/odoo-rpc.py b/odoo-rpc.py
index 636e621..944fe4c 100755
--- a/odoo-rpc.py
+++ b/odoo-rpc.py
@@ -323,8 +323,8 @@ def flatten_stack( data ):
 		if type(data[key]) != dict:
 			flat.append(key)
 		else:
-			result += flatten_stack(data[key])
-	return result
+			flat += flatten_stack(data[key])
+	return flat