diff --git a/src/rougail/object_model.py b/src/rougail/object_model.py
index c5d6e5f37..d26dcb8d9 100644
--- a/src/rougail/object_model.py
+++ b/src/rougail/object_model.py
@@ -338,6 +338,7 @@ class VariableCalculation(Calculation):
     variable: StrictStr
     propertyerror: bool = True
     allow_none: bool = False
+    optional: bool = False
 
     def to_function(
         self,