The following code is from the main content not from the exercise.
combined = pd.concat([happiness2015, happiness2016, happiness2017], ignore_index=True) combined.isnull().sum()
The expected output looks like this:
However, my output looks like this:
The same problem persists with the next lesson:
The code to generate heatmap looks like this:
import seaborn as sns combined_updated = combined.set_index('YEAR') sns.heatmap(combined_updated.isnull(), cbar=False)