Error when no standard name in FirstOccurrence

Describes first bullet point of !52 (closed) :

iris throws the error below when the standard_name is None (I have tried removing the dict entry, as well as setting it to '' to similar effect)

(climix-devel) ~/CODE/climix/data >climix -x faf /home/rossby/prod/201116/netcdf/day/tasmin_EUR-44_ECMWF-ERAINT_evaluation_r1i1p1_SMHI-RCA4_v1_day_*.nc
Cluster ready; starting main program.
Traceback (most recent call last):
  File "/home/sm_lbarr/.conda/envs/climix-devel/bin/climix", line 11, in <module>
    load_entry_point('climix', 'console_scripts', 'climix')()
  File "/home/sm_lbarr/CODE/climix/climix/main.py", line 182, in main
    args.output_template, args.sliced_mode)
  File "/home/sm_lbarr/CODE/climix/climix/main.py", line 169, in do_main
    result = index(input_data, sliced_mode=sliced_mode)
  File "/home/sm_lbarr/CODE/climix/climix/index.py", line 22, in __call__
    aggregated = sub_cube.aggregated_by(coord_name, self.aggregator)
  File "/home/sm_lbarr/.conda/envs/climix-devel/lib/python3.7/site-packages/iris/cube.py", line 3473, in aggregated_by
    coords, **kwargs)
  File "/home/sm_lbarr/CODE/climix/climix/aggregators.py", line 30, in post_process
    standard_name = self.output_metadata['standard_name']
KeyError: 'standard_name'

@a002160 I need you iris insight on this one ....