diff --git a/src/rougail/object_model.py b/src/rougail/object_model.py
index 03f429cbd..cab67681d 100644
--- a/src/rougail/object_model.py
+++ b/src/rougail/object_model.py
@@ -369,6 +369,7 @@ class VariableCalculation(Calculation):
     variable: StrictStr
     propertyerror: bool = True
     allow_none: bool = False
+    optional: bool = False
 
     def get_params(self,
                    objectspace,