Issue with stopmarketorder

Hi
I have an issue with the stopmarketorder method. My code looks like below 

    def OnData(self, slice: Slice)-> None:
      self.RollContract()


      price = self.Securities[self._continuousContract.Symbol].Price
      high = self.Securities[self._continuousContract.Symbol].high
      low = self.Securities[self._continuousContract.Symbol].low
      open = self.Securities[self._continuousContract.Symbol].open


      for changedEvent in slice.SymbolChangedEvents.Values:
           if changedEvent.Symbol == self._continuousContract.Symbol:
               self.Log(f"SymbolChanged event: {changedEvent}")


     


      invested = self.check_for_existing_orders_or_positions()


      if not invested:
         if slice.time.hour == 9 and slice.time.minute == 31:
            buyorder = self.stop_market_order(self._continuousContract.Mapped, 2, high+0.25)
            sellorder = self.stop_market_order(self._continuousContract.Mapped, -2, low-0.25)  

but i keep getting this error:
Runtime Error: Value cannot be null. (Parameter 'key')
  at OnData
    buyorder = self.stop_market_order(self._continuousContract.Mapped, 2, high+0.25)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 in main.py: line 45 (Open Stack Trace)

I am trying to pass in the high + 1 tick, therefore the 0.25 as this is tested on ES. 
What am i doing wrong ?

Thanks in advance.