diff --git a/weighing.py b/weighing.py
index 021500b..810cdbe 100644
--- a/weighing.py
+++ b/weighing.py
@@ -415,7 +415,8 @@ class Weighing(Workflow, ModelSQL, ModelView):
         cls.analysis(to_analysis)
 
     def get_not_assigned_weight(self, name):
-        return self.netweight - sum([p.netweight for p in self.parcels])
+        if self.netweight:
+            return self.netweight - sum([p.netweight or 0 for p in self.parcels])
 
     @classmethod
     @ModelView.button