ValueError: 21 columns passed, passed data had 20 columns

My Code is given below :

import seaborn as sn
import pandas as pd
import matplotlib.pyplot as plt
array = confusion_matrix

df_cm = pd.DataFrame(array, index = [i for i in names],
              columns = [i for i in names])
df_norm_col=(df_cm-df_cm.mean())/df_cm.std()
#df_norm_col=df_cm-df_cm.mean()
plt.figure(figsize = (15,10))
sn.heatmap(df_norm_col, annot=True,cmap='viridis')

and I get the following error:

ValueError: 21 columns passed, passed data had 20 columns