Hi, I'm trying to write dataframe to MariaDB.
I got this error message, but I have no clue.
Please give some advice.

Py4JJavaErrorTraceback (most recent call last)
<ipython-input-64-300efd2c6967> in <module>()
----> 1 result1.filter(result1["gnl_nm_set"] == "").count()

/usr/local/linewalks/spark/spark/python/pyspark/sql/dataframe.pyc in count(self)
    297         2
    298         """
--> 299         return int(self._jdf.count())
    300 
    301     @ignore_unicode_prefix

/usr/local/linewalks/spark/spark/python/lib/py4j-0.10.1-src.zip/py4j/java_gateway.py in __call__(self, *args)
    931         answer = self.gateway_client.send_command(command)
    932         return_value = get_return_value(
--> 933             answer, self.gateway_client, self.target_id, self.name)
    934 
    935         for temp_arg in temp_args:

/usr/local/linewalks/spark/spark/python/pyspark/sql/utils.pyc in deco(*a, **kw)
     61     def deco(*a, **kw):
     62         try:
---> 63             return f(*a, **kw)
     64         except py4j.protocol.Py4JJavaError as e:
     65             s = e.java_exception.toString()

/usr/local/linewalks/spark/spark/python/lib/py4j-0.10.1-src.zip/py4j/protocol.py in get_return_value(answer, gateway_client, target_id, name)
    310                 raise Py4JJavaError(
    311                     "An error occurred while calling {0}{1}{2}.\n".
--> 312                     format(target_id, ".", name), value)
    313             else:
    314                 raise Py4JError(

Py4JJavaError: An error occurred while calling o1331.count.
: org.apache.spark.SparkException: Job aborted due to stage failure: Task 178 in stage 324.0 failed 4 times, most recent failure: Lost task 178.3 in stage 324.0 (TID 14274, 7.linewalks.local): org.apache.spark.api.python.PythonException: Traceback (most recent call last):
  File "/home/hadoop/hdtmp/nm-local-dir/usercache/hadoop/appcache/application_1491889279272_0040/container_1491889279272_0040_01_000003/pyspark.zip/pyspark/worker.py", line 172, in main
    process()
  File "/home/hadoop/hdtmp/nm-local-dir/usercache/hadoop/appcache/application_1491889279272_0040/container_1491889279272_0040_01_000003/pyspark.zip/pyspark/worker.py", line 167, in process
    serializer.dump_stream(func(split_index, iterator), outfile)
  File "/home/hadoop/hdtmp/nm-local-dir/usercache/hadoop/appcache/application_1491889279272_0040/container_1491889279272_0040_01_000003/pyspark.zip/pyspark/worker.py", line 106, in <lambda>
    func = lambda _, it: map(mapper, it)
  File "/home/hadoop/hdtmp/nm-local-dir/usercache/hadoop/appcache/application_1491889279272_0040/container_1491889279272_0040_01_000003/pyspark.zip/pyspark/worker.py", line 92, in <lambda>
    mapper = lambda a: udf(*a)
  File "/home/hadoop/hdtmp/nm-local-dir/usercache/hadoop/appcache/application_1491889279272_0040/container_1491889279272_0040_01_000003/pyspark.zip/pyspark/worker.py", line 70, in <lambda>
    return lambda *a: f(*a)
  File "<ipython-input-22-457936c8f59f>", line 3, in <lambda>
TypeError: sequence item 0: expected string, NoneType found