loc API gives KeyError: "not all values found in index"

I am having issues with the loc API of xarray. I am trying to select data that satisfy a certain condition. Below should be a reproducible example:

import xarray as xr
ds = xr.tutorial.load_dataset('air_temperature')
da = ds.air
mask = 2.3e2*xr.ones_like(da[0,0])
da[0,0].loc[da[0,0]<mask]

but this gives the following error:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-238-75b6b6f544d4> in <module>
      1 da = ds.air
      2 mask = 2.3e2*xr.ones_like(da[0,0])
----> 3 da[0,0].loc[da[0,0]<mask]

~/miniconda3/envs/ensemble/lib/python3.7/site-packages/xarray/core/dataarray.py in __getitem__(self, key)
    194             labels = indexing.expanded_indexer(key, self.data_array.ndim)
    195             key = dict(zip(self.data_array.dims, labels))
--> 196         return self.data_array.sel(**key)
    197 
    198     def __setitem__(self, key, value) -> None:

~/miniconda3/envs/ensemble/lib/python3.7/site-packages/xarray/core/dataarray.py in sel(self, indexers, method, tolerance, drop, **indexers_kwargs)
   1045             method=method,
   1046             tolerance=tolerance,
-> 1047             **indexers_kwargs
   1048         )
   1049         return self._from_temp_dataset(ds)

~/miniconda3/envs/ensemble/lib/python3.7/site-packages/xarray/core/dataset.py in sel(self, indexers, method, tolerance, drop, **indexers_kwargs)
   1998         indexers = either_dict_or_kwargs(indexers, indexers_kwargs, "sel")
   1999         pos_indexers, new_indexes = remap_label_indexers(
-> 2000             self, indexers=indexers, method=method, tolerance=tolerance
   2001         )
   2002         result = self.isel(indexers=pos_indexers, drop=drop)

~/miniconda3/envs/ensemble/lib/python3.7/site-packages/xarray/core/coordinates.py in remap_label_indexers(obj, indexers, method, tolerance, **indexers_kwargs)
    390 
    391     pos_indexers, new_indexes = indexing.remap_label_indexers(
--> 392         obj, v_indexers, method=method, tolerance=tolerance
    393     )
    394     # attach indexer's coordinate to pos_indexers

~/miniconda3/envs/ensemble/lib/python3.7/site-packages/xarray/core/indexing.py in remap_label_indexers(data_obj, indexers, method, tolerance)
    259             coords_dtype = data_obj.coords[dim].dtype
    260             label = maybe_cast_to_coords_dtype(label, coords_dtype)
--> 261             idxr, new_idx = convert_label_indexer(index, label, dim, method, tolerance)
    262             pos_indexers[dim] = idxr
    263             if new_idx is not None:

~/miniconda3/envs/ensemble/lib/python3.7/site-packages/xarray/core/indexing.py in convert_label_indexer(index, label, index_name, method, tolerance)
    191             indexer = get_indexer_nd(index, label, method, tolerance)
    192             if np.any(indexer < 0):
--> 193                 raise KeyError("not all values found in index %r" % index_name)
    194     return indexer, new_index
    195 

KeyError: "not all values found in index 'lat'"

Since the mask is identical to the xarray.DataArray da in terms of dimensions and coordinates, I don't think this error makes sense... If I change the coordinates in the following manner:

da = xr.DataArray(ds.air.data, dims=ds.air.dims, 
                  coords={'time':range(len(ds.time)),'lat':range(len(ds.lat)),'lon':range(len(ds.lon))})
mask = da[0,0] < 2.3e2*xr.ones_like(da[0,0])
da[0,0].loc[mask]

it works:

<xarray.DataArray (lon: 5)>
array([228.79999, 227.29999, 227.     , 227.5    , 228.79999],
      dtype=float32)
Coordinates:
    time     int64 0
    lat      int64 0
  * lon      (lon) int64 44 45 46 47 48

Am I missing something or is this possibly a bug? Thank you in advance for your help.

Reproduce the reported example using xarray's loc API, then compare it with the version using integer coordinates. Start from the loc/sel indexer path shown in the traceback and investigate why the labeled mask is treated as missing index values. Done means the original coordinate example selects the expected data without raising KeyError.