`np.array([pd.Series({'a':'a'})])` works but `np.array([pd.Series({1:1})])` doesn't
The last line here throws a `KeyError: 0`
```
import numpy as np
import pandas as pd
np.array([pd.Series({0:0})])
np.array([pd.Series({'a':'a'})])
np.array([pd.Series({1:1})])
```
Is that a bug?