About "fillna()"...kind like a bug?
Actually I wanna replace NaN with "mode" value of the variable.
But the following code doesn't work.
```python
# Your code here
a = df.Embarked.mode()
df.Embarked[df.Embarked.isna()].replace(a.values[0], inplace=True)
df.Embarked[df.Embarked.isna()]
>>>61     NaN
829    NaN
Name: Embarked, dtype: object
```
