# Buggy code
def fetch_chunk_data(self, session_id, chunk_key, index_obj=None):
    endpoints = self.chunk_meta_client.get_workers(session_id, chunk_key)
    if endpoints is None:
        raise KeyError(f"Chunk key {chunk_key} not exist in cluster")
    sender_ref = self.actor_client.actor_ref(
        ResultSenderActor.default_uid(), address=random.choice(endpoints)
    )
    return sender_ref.fetch_data(session_id, chunk_key, index_obj, _wait=False)


# Traceback (TypeError)
Traceback (most recent call last):
File "/Users/wenjun.swj/miniconda3/lib/python3.8/site-packages/IPython/core/interactiveshell.py", line 3417, in run_code
exec(code_obj, self.user_global_ns, self.user_ns)
File "<ipython-input-9-60fd09690beb>", line 1, in <module>
df[df[0] > 0.5].execute()
File "/Users/wenjun.swj/Code/mars/mars/core.py", line 643, in execute
return run()
File "/Users/wenjun.swj/Code/mars/mars/core.py", line 639, in run
self.data.execute(session, **kw)
File "/Users/wenjun.swj/Code/mars/mars/core.py", line 379, in execute
return run()
File "/Users/wenjun.swj/Code/mars/mars/core.py", line 374, in run
session.run(self, **kw)
File "/Users/wenjun.swj/Code/mars/mars/session.py", line 505, in run
result = self._sess.run(*tileables, **kw)
File "/Users/wenjun.swj/Code/mars/mars/session.py", line 111, in run
res = self._executor.execute_tileables(tileables, **kw)
File "/Users/wenjun.swj/Code/mars/mars/utils.py", line 451, in _inner
return func(*args, **kwargs)
File "/Users/wenjun.swj/Code/mars/mars/executor.py", line 860, in execute_tileables
chunk_graph = chunk_graph_builder.build(
File "/Users/wenjun.swj/Code/mars/mars/utils.py", line 451, in _inner
return func(*args, **kwargs)
File "/Users/wenjun.swj/Code/mars/mars/tiles.py", line 347, in build
chunk_graph = super().build(
File "/Users/wenjun.swj/Code/mars/mars/utils.py", line 451, in _inner
return func(*args, **kwargs)
File "/Users/wenjun.swj/Code/mars/mars/tiles.py", line 262, in build
self._on_tile_failure(tileable_data.op, exc_info)
File "/Users/wenjun.swj/Code/mars/mars/tiles.py", line 301, in inner
raise exc_info[1].with_traceback(exc_info[2]) from None
File "/Users/wenjun.swj/Code/mars/mars/tiles.py", line 242, in build
tiled = self._tile(tileable_data, tileable_graph)
File "/Users/wenjun.swj/Code/mars/mars/tiles.py", line 337, in _tile
return super()._tile(tileable_data, tileable_graph)
File "/Users/wenjun.swj/Code/mars/mars/tiles.py", line 201, in _tile
tds[0]._inplace_tile()
File "/Users/wenjun.swj/Code/mars/mars/core.py", line 168, in _inplace_tile
return handler.inplace_tile(self)
File "/Users/wenjun.swj/Code/mars/mars/tiles.py", line 136, in inplace_tile
dispatched = self.dispatch(to_tile.op)
File "/Users/wenjun.swj/Code/mars/mars/utils.py", line 451, in _inner
return func(*args, **kwargs)
File "/Users/wenjun.swj/Code/mars/mars/tiles.py", line 119, in dispatch
tiled = op_cls.tile(op)
File "/Users/wenjun.swj/Code/mars/mars/dataframe/indexing/getitem.py", line 273, in tile
return cls.tile_with_mask(op)
File "/Users/wenjun.swj/Code/mars/mars/dataframe/indexing/getitem.py", line 287, in tile_with_mask
align_dataframe_series(in_df, mask, axis='index')
File "/Users/wenjun.swj/Code/mars/mars/dataframe/align.py", line 713, in align_dataframe_series
index_splits, index_nsplits = _calc_axis_splits(left.index_value, right.index_value,
File "/Users/wenjun.swj/Code/mars/mars/dataframe/align.py", line 490, in _calc_axis_splits
right_splits = left_splits = [[c] for c in left_chunk_index_min_max]
TypeError: 'NoneType' object is not iterable

# Fixed code
def fetch_chunk_data(self, session_id, chunk_key, index_obj=None):
    endpoints = self.chunk_meta_client.get_workers(session_id, chunk_key)
    if endpoints is None:
        raise KeyError(f"Chunk key {chunk_key} not exist in cluster")

    source_endpoint = random.choice(endpoints)
    logger.debug("Fetching chunk %s from worker %s", chunk_key, source_endpoint)
    sender_ref = self.actor_client.actor_ref(
        ResultSenderActor.default_uid(), address=source_endpoint
    )
    return sender_ref.fetch_data(session_id, chunk_key, index_obj, _wait=False)