diff --git a/ufo/module/basic.py b/ufo/module/basic.py
index dc7d391b..28463a7c 100644
--- a/ufo/module/basic.py
+++ b/ufo/module/basic.py
@@ -743,7 +743,7 @@ def experience_saver(self) -> None:
             summaries, os.path.join(experience_path, "experience_db")
         )
 
-        self.cost += cost
+        self.cost += cost or 0
         self.logger.info(f"The experience has been saved to {experience_path}")
 
     def print_cost(self) -> None:
@@ -848,7 +848,7 @@ def evaluation(self) -> None:
 
         self._results.append(result)
 
-        self.cost += cost
+        self.cost += cost or 0
 
         evaluator.print_response(result)