square-bracket slice a Dataset with a DataArray

Given this:

ds = xarray.Dataset(
    data_vars={
        'vote': ('pupil', [5, 7, 8]),
        'age': ('pupil', [15, 14, 16])
    },
    coords={
        'pupil': ['Alice', 'Bob', 'Charlie']
    })


<xarray.Dataset>
Dimensions:  (pupil: 3)
Coordinates:
  * pupil    (pupil) <U7 'Alice' 'Bob' 'Charlie'
Data variables:
    vote     (pupil) int64 5 7 8
    age      (pupil) int64 15 14 16

Why does this work:

ds.age[ds.vote >= 6]

<xarray.DataArray 'age' (pupil: 2)>
array([14, 16])
Coordinates:
  * pupil    (pupil) <U7 'Bob' 'Charlie'

But this doesn't?

ds[ds.vote >= 6]

KeyError: False

ds.vote >= 6 is a DataArray with dims=('pupil', ) and dtype=bool, so I can't think of any ambiguity in what I want to achieve?

Workaround:

ds.sel(pupil=ds.vote >= 6)


<xarray.Dataset>
Dimensions:  (pupil: 2)
Coordinates:
  * pupil    (pupil) <U7 'Bob' 'Charlie'
Data variables:
    vote     (pupil) int64 7 8
    age      (pupil) int64 14 16

Start from Dataset square-bracket indexing and compare it with the shown DataArray indexing and Dataset.sel examples. Confirm that a boolean DataArray such as ds.vote >= 6 selects matching entries across the Dataset, and verify the result against the expected two-row Dataset output.