diff --git a/algo/mm.py b/algo/mm.py
index 7ecd286..93adc67 100644
--- a/algo/mm.py
+++ b/algo/mm.py
@@ -66,8 +66,8 @@ class MM:
         logging.info(f"-cut (change: {change}, pos: {pos} -> 0)")
       if posCut:
         logging.info(f"+cut (change: {change}, pos: {pos} -> 0)")
-      self._lastBuy   = None
-      self._lastSell  = None
+      self._lastBuy   = ""
+      self._lastSell  = ""
       sell_amount = pos * lot
       if sell_amount > 0:
         await self._player.orderMarketSell(self._pair.names, amount=sell_amount)