error on stock indicator code on balance volume

[yatish]

# On-balance Volume  
def on_balance_volume(aapl, n):

    i = 0

    OBV = [0]
#I have a dataframe with open high low close volume of stock prices and wanted to calaculate obv of parameter 10 and 20
    while (i < (len(aapl.index)-1)):


        if aapl.iloc[i + 1]['Close'] - aapl.iloc[i]['Close'] > 0:
            OBV.append(aapl.iloc[i]['Volume'])
        elif aapl.iloc[i + 1]['Close'] - aapl.iloc[i]['Close'] < 0:
            OBV.append(-aapl.iloc[i + 1]['Volume'])
        else :
            OBV.append(0) 
        i = i + 1
       
        OBV = pd.Series(OBV)
    #print(obv)    
    #obv is calculated till here but not passed to the rolling function
        OBV_ma = pd.Series(OBV.rolling(n, min_periods=n).mean(), name='OBV_' + str(n))

        aapl = aapl.join(OBV_ma)

        return aapl

the output for obv is displaying nan values tried several other ways...please can someone check my whole code and guide me
I have dataframe of open high low close volume of stock prices