diff --git a/line.py b/line.py
index c266820..29ec22c 100644
--- a/line.py
+++ b/line.py
@@ -150,6 +150,8 @@ class EvaluationLine(ModelSQL, ModelView):
         Currency = Pool().get('currency.currency')
 
         exp = Decimal(Decimal(1) / 10 ** self.currency_digits)
+        if amount is None:
+            return Decimal('0.0')
         return Currency.compute(
                 from_currency,
                 amount,