docode_vertical_coords : The standard name for the 'depth' variable is not available.

[knutfrode]

Hi,

I am not able to get decode_vertical_coords to work, and it also fails in the documentation:
https://cf-xarray.readthedocs.io/en/stable/parametricz.html#decoding-parametric-coordinates

>>> from cf_xarray.datasets import romsds
>>> romsds.cf.decode_vertical_coords(outnames={'s_rho': 'z_rho'})  # adds new z_rho variable

Traceback (most recent call last):
  File "<python-input-0>", line 2, in <module>
    romsds.cf.decode_vertical_coords(outnames={'s_rho': 'z_rho'})  # adds new z_rho variable
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/knutfd/micromamba/envs/opendrift/lib/python3.13/site-packages/cf_xarray/accessor.py", line 3110, in decode_vertical_coords
    ds.coords[zname] = transform.from_terms(terms).decode()
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/home/knutfd/micromamba/envs/opendrift/lib/python3.13/site-packages/cf_xarray/parametric.py", line 636, in decode
    return z.assign_attrs(standard_name=self.computed_standard_name)
                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/knutfd/micromamba/envs/opendrift/lib/python3.13/site-packages/cf_xarray/parametric.py", line 641, in computed_standard_name
    return _derive_ocean_stdname(eta=self.eta.attrs, depth=self.depth.attrs)
  File "/home/knutfd/micromamba/envs/opendrift/lib/python3.13/site-packages/cf_xarray/parametric.py", line 81, in _derive_ocean_stdname
    raise ValueError(
        f"The standard name for the {x!r} variable is not available."
    ) from None
ValueError: The standard name for the 'depth' variable is not available.

Is something missing in the dataset or in the code?