diff --git a/stock.py b/stock.py
index 0c31b6a..a226ad1 100644
--- a/stock.py
+++ b/stock.py
@@ -33,7 +33,7 @@ class UnitLoad:
         return ''
 
     def get_grai(self):
-        return None
+        return ''
 
     @staticmethod
     def _sscc_control_digit(num):