Error when supplying a tuple of dimensions to DataArray.sortby()

What happened:

A KeyError is thrown when supplying a tuple of dimensions to DataArray.sortby()

What you expected to happen:

The dataarray to be sorted according to the dimensions.

Minimal Complete Verifiable Example:

import xarray as xr
import numpy as np

da=xr.DataArray(np.random.rand(3,3), coords=(('x', range(3, 0, -1)), ('y', range(3, 0, -1))))
da.sortby(da.dims)

Anything else we need to know?:

If the tuple is cast to a list it works correctly:

da.sortby(list(da.dims))

Environment:

Start by running the minimal example against DataArray.sortby(), comparing da.sortby(da.dims) with da.sortby(list(da.dims)). Trace the DataArray.sortby() entry point and add coverage showing that the tuple form sorts according to the dimensions without raising KeyError.