i keep getting the following error
ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
There was a runtime error on line 26.

LINE 26 - is the beginning of the if statement and is highlighted in bold. thank you in advance!!!!

import numpy as np

def initialize(context):
context.cost = sid(1787)

schedule_function(compute_z_score,date_rules.every_day(), time_rules.market_close(minutes=60))  

# Flags to tell us if we're currently in a trade  
context.long_on_spread = False  
context.shorting_spread = False  

def compute_z_score(context, data):
#for conveniece
costco = context.cost

#historical data  
prices = data.history(context.cost,"price",1,"1d")  

#create our cumulative moving average  
cumulative_moving_average = prices.expanding(min_periods=1).mean()  

#compute z score components  
std = prices.expanding(min_periods=1).std()  

#compute z score  
**if std > 0:**  
    z_score = (prices - cumulative_moving_average) / std  

    #entry case 1  
    if z_score > 1 and not context.shorting_spread:  
        order_target_percent(costco, -1)  
        context.shorting_spread = True  
        context.long_on_spread = False  
    elif z_score < -1 and not context.long_on_spread:  
        order_target_percent(costco, 1)  
        context.shorting_spread = False  
        context.long_on_spread = True  
    elif -0.5 < z_score < 0.5:  
        order_target_percent(costco, 0)  
        context.shorting_spread = False  
        context.long_on_spread = False  

    record('zscore', z_score)