isnull().sum() returning incorrect figure

I am doing a prediction using RandomForestRegressor.

Below is the code for test data:

    X = test_set.drop(['C_Nxt10DMx'], axis=1) # drop labels for training set
    Y = test_set['C_Nxt10DMx'].copy()                 
    test_prepared     = full_pipeline.transform(X)
    final_predictions = forest_reg.predict(test_prepared)
    final_mse = mean_squared_error(Y, final_predictions)
    final_rmse = np.sqrt(final_mse)
    (forest_rmse*100)/Y.mean()

I am printing Y.isnull().sum() and it is returning a 0 value. However, when I trying to bind the value in a data frame, it is returning an incorrect figure.

df =  pd.DataFrame() 
df['pred'] = final_predictions
df['Label'] = Y
print(Y.isnull().sum())
print(df['Label'].isnull().sum())

# 7646