hv.timeseries.rolling doesn't work with object typed groupby

[ahuang11]

import xarray as xr
import hvplot.pandas
from holoviews.operation import timeseries

ds = xr.tutorial.open_dataset("air_temperature")
df = ds.mean(dim=["lat", "lon"]).to_dataframe()
df["year"] = df.index.strftime("%b %Y")

curve = df.hvplot("time", "air", groupby="year").opts(color="black", title="Outlier Detection")
curve_rolling = timeseries.rolling(curve.clone(label="Rolling Mean"))
curve_rolling

Change "%b %Y" to "%Y" and it works.