diff --git a/modules/purchase_trade/fee.py b/modules/purchase_trade/fee.py
index 2b197fe..d46284b 100755
--- a/modules/purchase_trade/fee.py
+++ b/modules/purchase_trade/fee.py
@@ -124,7 +124,7 @@ class Fee(ModelSQL,ModelView):
             return qt
         else:
             if self.auto_calculation:
-                return (qt * unit.factor / Decimal(self.unit.factor)).to_integral_value(rounding=ROUND_UP)
+                return (qt * Decimal(unit.factor) / Decimal(self.unit.factor)).to_integral_value(rounding=ROUND_UP)
 
     @fields.depends('mode','_parent_line.lots','_parent_sale_line.lots')
     def on_change_with_unit(self, name=None):