I would like to have a nested dictionary with a list of values that are attached to the sub-key. Can't get the sub-key to recognise.

```py
month = {}
    for row in date:
        month.setdefault(row[1],{row[0]: []})
        month[row[0]].append(row[4])

print(month[main_key][sub_key])

```

I expect a list of values to be populated out through the appending method. However, I keep getting a keyError.

The desired output:

```
{'row[1]': {'row[0]' :[row[4], row[4], row[4]],
            'row[0]' :[row[4], row[4], row[4]]}}

```