diff --git a/product.py b/product.py
index bc22b8d580..9e6dc1a692 100644
--- a/product.py
+++ b/product.py
@@ -529,7 +529,7 @@ class ProductQuantitiesByWarehouse(ModelSQL, ModelView):
             key = trans_context['product_template']
         warehouse_id = trans_context.get('warehouse')
 
-        dates = sorted(l.date for l in lines)
+        dates = sorted({l.date for l in lines})
         quantities = {}
         date_start = None
         for date in dates:
@@ -538,6 +538,7 @@ class ProductQuantitiesByWarehouse(ModelSQL, ModelView):
                 'stock_date_end': date,
                 'forecast': True,
                 }
+            assert date != date_start
             with Transaction().set_context(**context):
                 quantities[date] = Product.products_by_location(
                     [warehouse_id],