Every Window Empty

import numpy as np, pandas as pd

s = pd.Series([np.nan, 1.0, 2.0, np.nan, np.nan, 3.0])
print(s.rolling(3).sum().tolist())

What prints?

[nan, nan, 3.0, 3.0, nan, nan]

[nan, nan, nan, nan, nan, nan]

[nan, 1.0, 3.0, 3.0, 2.0, 3.0]

[nan, nan, 3.0, 3.0, 2.0, 3.0]