Why am I getting NaN values for the data frame df_std_plus

Question

I would like to know why I am getting NaN values for df_std_plus

df_data_weekly = (df_data['ISO week-counting year'] != 2021) & (df_data['ISO week-counting year'] != 2020)

df_data_2015_2019 = df_data[df_data_weekly]

df_traffic_baseline1 = df_data_2015_2019.groupby('week')

df_traffic_baseline = df_traffic_baseline1.agg({'RWS01_MONIBAS_0101hrr0294ra':['mean','std']})

df_traffic_average = df_traffic_baseline1.agg({'RWS01_MONIBAS_0101hrr0294ra':['mean']})
df_traffic_std = df_traffic_baseline1.agg({'RWS01_MONIBAS_0101hrr0294ra':['std']})
df_std_plus = (df_traffic_average) + (df_traffic_std)

#ax_combined = df_traffic_average['RWS01_MONIBAS_0101hrr0294ra'].plot(linewidth=4);
#df_std_plus.plot(ax=ax_combined);

print(df_std_plus)

output df_std_plus