UnpicklingError while using spark streaming

[lovemoon]

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spark2.1.1 & python2.7.11

I want to union another rdd in Dstream.transform() like below:

sc = SparkContext()
ssc = StreamingContext(sc, 1)

init_rdd = sc.textFile('file:///home/zht/PycharmProjects/test/text_file.txt')

lines = ssc.socketTextStream('localhost', 9999)

lines = lines.transform(lambda rdd: rdd.union(init_rdd))

lines.pprint()

ssc.start()
ssc.awaitTermination()


And get error about pickle:

org.apache.spark.api.python.PythonException: Traceback (most recent call last):
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/worker.py", line 174, 
in main
    process()
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/worker.py", line 169, 
in process
    serializer.dump_stream(func(split_index, iterator), outfile)
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 
268, in dump_stream
    vs = list(itertools.islice(iterator, batch))
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/rdd.py", line 1338, in 
takeUpToNumLeft
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 
144, in load_stream
    yield self._read_with_length(stream)
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 
169, in _read_with_length
    return self.loads(obj)
  File "/usr/local/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 
454, in loads
    return pickle.loads(obj)
UnpicklingError: unpickling stack underflow


text_file.txt only include one letter :

 a
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