Hello, Per the [documentation](http://dask.pydata.org/en/latest/dataframe-api.html#dask.dataframe.core.Series.notnull), `series.notnul()`l has a boolean inverse `series.isnull()`. Sadly, however, that's not the case. (Trivial I know.)