>>> import cfdm
>>> cfdm.environment(paths=False)
Platform: Linux-5.4.0-53-generic-x86_64-with-debian-bullseye-sid
HDF5 library: 1.10.5
netcdf library: 4.6.3
python: 3.7.0
netCDF4: 1.5.4
cftime: 1.3.0
numpy: 1.18.4
netcdf_flattener: 1.2.0
cfdm: 1.8.7.0
>>> import cfdm; f = cfdm.read('~/thetao_Omon_MIROC6_abrupt-4xCO2_r1i1p1f1_gn_344001-344912.cdl')[0]
>>> cfdm.write(f, 'delme.nc')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-e145e8c98469> in <module>()
----> 1 cfdm.write(f, 'delme.nc')

~/cfdm/cfdm/read_write/write.py in write(fields, filename, fmt, overwrite, global_attributes, variable_attributes, file_descriptors, external, Conventions, datatype, least_significant_digit, endian, compress, fletcher32, shuffle, string, verbose, warn_valid, group, coordinates, _implementation)
    460                      string=string, verbose=verbose,
    461                      warn_valid=warn_valid, group=group,
--> 462                      coordinates=coordinates, extra_write_vars=None)

~/cfdm/cfdm/decorators.py in verbose_override_wrapper(self, *args, **kwargs)
    184         # enabling
    185         try:
--> 186             return method_with_verbose_kwarg(self, *args, **kwargs)
    187         except Exception:
    188             raise

~/cfdm/cfdm/read_write/netcdf/netcdfwrite.py in write(self, fields, filename, fmt, overwrite, global_attributes, variable_attributes, file_descriptors, external, Conventions, datatype, least_significant_digit, endian, compress, fletcher32, shuffle, scalar, string, extra_write_vars, verbose, warn_valid, group, coordinates)
   4466         # ------------------------------------------------------------
   4467         for f in fields:
-> 4468             self._write_field(f)
   4469 
   4470         # ------------------------------------------------------------

~/cfdm/cfdm/read_write/netcdf/netcdfwrite.py in _write_field(self, f, add_to_seen, allow_data_insert_dimension)
   3390         for key, anc in sorted(
   3391                 self.implementation.get_domain_ancillaries(f).items()):
-> 3392             self._write_domain_ancillary(f, key, anc)
   3393 
   3394         # ------------------------------------------------------------

~/cfdm/cfdm/read_write/netcdf/netcdfwrite.py in _write_domain_ancillary(self, f, key, anc)
   2192 
   2193             # Create a new domain ancillary variable
-> 2194             self._write_netcdf_variable(ncvar, ncdimensions, anc)
   2195 
   2196         g['key_to_ncvar'][key] = ncvar

~/cfdm/cfdm/read_write/netcdf/netcdfwrite.py in _write_netcdf_variable(self, ncvar, ncdimensions, cfvar, omit, extra, fill, data_variable)
   2533         if g['group']:
   2534             groups = self._groups(ncvar)
-> 2535             for ncdim in ncdimensions:
   2536                 ncdim_groups = self._groups(ncdim)
   2537                 if not groups.startswith(ncdim_groups):

TypeError: 'NoneType' object is not iterable