Python 3 pipeline fails with errors in StockUnpickler.find_class() during loading a main session.

class FlattenCustomActions(beam.PTransform):
    """ Transforms Facebook Day Actions        Only retains actions with custom_conversions
        Flattens the actions
        Adds custom conversions names using a side input
    """    
    def __init__(self, conversions):
        super(FlattenCustomActions, self).__init__()
        self.conversions = conversions    def expand(self, input_or_inputs):
        return (
            input_or_inputs
            | "FlattenActions" >> beam.ParDo(flatten_filter_actions)
            | "AddConversionName" >> beam.Map(add_conversion_name, self.conversions)
        )

# ...
# in run():
    pipeline_options = PipelineOptions(pipeline_args)
    pipeline_options.view_as(SetupOptions).save_main_session = True
    p = beam.Pipeline(options=pipeline_options)
    conversions_output = (
        p
        | "ReadConversions" >> ReadFromText(known_args.input_conversions, coder=JsonCoder())
        | TransformConversionMetadata()
    )    (
        conversions_output
        | "WriteConversions"
        >> WriteCoerced(
            known_args.output_conversions,
            known_args.output_type,
            schema_path=BIGQUERY_SCHEMA_CONVERSIONS_PATH,
        )
    )    (
        p
        | ReadFacebookJson(known_args.input, retain_root_fields=True)
        | FlattenCustomActions(beam.pvalue.AsList(conversions_output))
        | "WriteActions"
        >> WriteCoerced(
            known_args.output, known_args.output_type, schema_path=BIGQUERY_SCHEMA_ACTIONS_PATH
        )
    )

Traceback (most recent call last): 
  File "/usr/local/lib/python3.6/site-packages/dataflow_worker/batchworker.py", line 773, in run self._load_main_session(self.local_staging_directory) 
  File "/usr/local/lib/python3.6/site-packages/dataflow_worker/batchworker.py", line 489, in _load_main_session pickler.load_session(session_file) 
  File "/usr/local/lib/python3.6/site-packages/apache_beam/internal/pickler.py", line 287, in load_session return dill.load_session(file_path) 
  File "/usr/local/lib/python3.6/site-packages/dill/_dill.py", line 410, in load_session module = unpickler.load() 
  File "/usr/local/lib/python3.6/site-packages/dill/_dill.py", line 474, in find_class return StockUnpickler.find_class(self, module, name) AttributeError: Can't get attribute 'FlattenCustomActions' on <module 'dataflow_worker.start' from '/usr/local/lib/python3.6/site-packages/dataflow_worker/start.py'>