diff -r 8a6a82c717d6 stock.py
--- a/trytond/trytond/modules/stock_lot/stock.py	Tue Jul 10 12:57:12 2018 +0200
+++ b/trytond/trytond/modules/stock_lot/stock.py	Tue Jul 10 12:58:45 2018 +0200
@@ -159,6 +159,20 @@
                 'lot_required': 'Lot is required for move of product "%s".',
                 })

+    @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))
+
     def check_lot(self):
         "Check if lot is required"
         if (self.state == 'done'