diff -r 366b4e8da556 trytond/trytond/modules/stock/location.py
--- a/trytond/trytond/modules/stock/location.py	Wed Apr 10 14:21:58 2019 +0200
+++ b/trytond/trytond/modules/stock/location.py	Wed Apr 10 17:42:42 2019 +0200
@@ -355,11 +355,14 @@
         if trans_context.get('product') is not None:
             grouping = ('product',)
             grouping_filter = ([trans_context['product']],)
-            key = trans_context['product']
+            key = (trans_context['product'],)
+            if trans_context.get('lot') is not None:
+                grouping += ('lot', )
+                key += (trans_context['lot'],)
         else:
             grouping = ('product.template',)
             grouping_filter = ([trans_context['product_template']],)
-            key = trans_context['product_template']
+            key = (trans_context['product_template'],)
         pbl = {}
         for sub_locations in grouped_slice(locations):
             location_ids = [l.id for l in sub_locations]
@@ -370,7 +373,7 @@
                         grouping_filter=grouping_filter,
                         with_childs=trans_context.get('with_childs', True)))

-        return dict((loc.id, pbl.get((loc.id, key), 0)) for loc in locations)
+        return dict((loc.id, pbl.get((loc.id,) + key, 0)) for loc in locations)

     @classmethod
     def search_quantity(cls, name, domain):