I have the following test code:

def handle_data(context, data):  
    for stock in context.stocks:  
        h = history(bar_count=1,frequency="1d",field="high")  
        l = history(bar_count=1,frequency="1d",field="low")  
        # Check if we are already in a position.  
        pos = context.portfolio.positions[stock]  
        # Handle open positions  
        if pos.amount != 0:  
            # Check stop-loss value and close if breached.  
            if data[stock].price/pos.cost_basis-1 > context.max_loss:  
                order_target(stock,0)  
                context.open_trade_dt[stock.sid] = None  
            # Check if we have passed the max duration to take profit/loss.  
            if data[stock].datetime - context.open_trade_dt[stock.sid] > dt.timedelta(hours=context.max_duration):  
                order_target(stock,0)  
                context.open_trade_dt[stock.sid] = None  
        # Or see if we meet Long trade criteria  
        elif data[stock].price > h[stock][0]:  
            order(stock,int(context.max_pos_value/data[stock].price))  
            context.open_trade_dt[stock.sid] = dt.datetime.now()  
        elif data[stock].price < l[stock][0]:  
            order(stock,-int(context.max_pos_value/data[stock].price))  
            context.open_trade_dt[stock.sid] = dt.datetime.now()  

The Build is failing with:

2   Error   Runtime exception: TypeError: unsupported operand type(s) for -: 'Timestamp' and 'NoneType'  

But the assignment is clearly made when the code actually opens a trade. Any reason for this?