TypeError: cannot do label indexing on <class 'pandas.core.indexes.base.Index'> with these indexers [nan] of <class 'float'>
```py
import pandas as pd
import numpy as np

df = pd.Series([1, '{1,2}', 1, None])
vc = df.value_counts(dropna=False)
print(vc.loc[np.nan])
print(vc[np.nan])
```

```
TypeError: cannot do label indexing on <class 'pandas.core.indexes.base.Index'> with these indexers [nan] of <class 'float'>
```

Hello, now I have trouble  getting null value count, help.
