Index: stock.py
===================================================================

--- .a/trytond/trytond/modules/stock_lot/stock.py
+++ .b/trytond/trytond/modules/stock_lot/stock.py
@@ -80,6 +80,20 @@
         for move in moves:
             move.check_lot()

+    @classmethod
+    def assign_try(cls, moves, with_childs=True, grouping=('product',)):
+        lot_moves = []
+        product_moves = []
+        for move in moves:
+            if move.lot:
+                lot_moves.append(move)
+            else:
+                product_moves.append(move)
+        return (super(Move, cls).assign_try(lot_moves,
+                with_childs=with_childs, grouping=('product', 'lot'))
+            & super(Move, cls).assign_try(product_moves,
+                with_childs=with_childs, grouping=grouping))
+