MT5 error 10013: Invalid request

void MoveStops(const ulong ticket, const double sl, const double tp)
  {
   MqlTradeRequest req;                     // never zeroed: the other fields hold leftovers
   MqlTradeResult  res;
   req.action = TRADE_ACTION_SLTP;
   req.symbol = _Symbol;
   req.sl     = sl;
   req.tp     = tp;                         // and no position ticket
   if(!OrderSend(req, res))
      Print("OrderSend failed: ", res.retcode);
  }

void MoveStops(const ulong ticket, const double sl, const double tp)
  {
   MqlTradeRequest     req = {};
   MqlTradeResult      res = {};
   MqlTradeCheckResult check = {};
   req.action   = TRADE_ACTION_SLTP;
   req.position = ticket;                   // which position to change
   req.symbol   = _Symbol;
   req.sl       = sl;
   req.tp       = tp;
   if(!OrderCheck(req, check))
     {
      PrintFormat("OrderCheck: %u %s", check.retcode, check.comment);
      return;
     }
   if(!OrderSend(req, res))
      Print("OrderSend failed: ", res.retcode, " ", res.comment);
  }