KeyError: 'shape_pt_sequence' when trying to access feed.shapes

[tmein]

I am loading Transperth's GTFS data using this function:
loaded_feed = gtfs_functions.Feed(feed_path, time_windows=[0, 24], busiest_date=False, patterns=False)
and then trying to access loaded_feed.shapes. This throws the following exception:

  File "[snip]trunk\open_data_toolset\ingestor\__main__.py", line 15, in <module>
    data_processing.process(input_file=args.input, force=args.f)
  File "[snip]trunk\open_data_toolset\ingestor\data_processing\__init__.py", line 52, in process
    process_tool.get(data_tool, lambda *args: unknown_tool_error(data_tool))(config, force)
  File "[snip]trunk\open_data_toolset\ingestor\data_processing\gtfs.py", line 175, in process
    trips = parsing.load_gtfs_trips(os.path.join(base_path, location))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[snip]trunk\open_data_toolset\ingestor\data_processing\parsing.py", line 334, in load_gtfs_trips
    data = data.join(loaded_feed.shapes.set_index('shape_id'), on='shape_id', how='inner')
                     ^^^^^^^^^^^^^^^^^^
  File "[snip]venv-test\Lib\site-packages\gtfs_functions\gtfs_functions.py", line 213, in shapes
    self._shapes = self.get_shapes()
                   ^^^^^^^^^^^^^^^^^
  File "[snip]venv-test\Lib\site-packages\gtfs_functions\gtfs_functions.py", line 726, in get_shapes
    aux.sort_values(['shape_id','shape_pt_sequence'], inplace=True)
  File "[snip]venv-test\Lib\site-packages\pandas\util\_decorators.py", line 331, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "[snip]venv-test\Lib\site-packages\pandas\core\frame.py", line 6894, in sort_values
    keys = [self._get_label_or_level_values(x, axis=axis) for x in by]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[snip]venv-test\Lib\site-packages\pandas\core\frame.py", line 6894, in <listcomp>
    keys = [self._get_label_or_level_values(x, axis=axis) for x in by]
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[snip]venv-test\Lib\site-packages\pandas\core\generic.py", line 1850, in _get_label_or_level_values
    raise KeyError(key)
KeyError: 'shape_pt_sequence'

I'm not too familiar with GTFS, but I ran the data through https://gtfs-validator.mobilitydata.org and didn't receive any errors, and looking at shapes.txt shape_pt_sequence does appear to be there and populated. My code works with a variety of other Australian GTFS feeds and I couldn't find any obvious differences in their shapes.txt (although I may well be looking in the wrong place)

I'm using gtfs-functions 2.5.