I'm new of this chainer,Is problem my vision of python or any other problem ?I doon't know what should i do ?could you help me ?

Load segmentation weights
Push Model to GPU
Main Begins
Exception in thread prefetch_loop:
Traceback (most recent call last):
File "/home/lvv/anaconda2/lib/python2.7/threading.py", line 801, in __bootstrap_inner
self.run()
File "/home/lvv/anaconda2/lib/python2.7/threading.py", line 754, in run
self.__target(*self.__args, **self.__kwargs)
File "/home/lvv/anaconda2/lib/python2.7/site-packages/chainer/iterators/multiprocess_iterator.py", line 336, in _run
alive = self._task()
File "/home/lvv/anaconda2/lib/python2.7/site-packages/chainer/iterators/multiprocess_iterator.py", line 355, in _task
data_all = future.get(_response_time)
File "/home/lvv/anaconda2/lib/python2.7/multiprocessing/pool.py", line 572, in get
raise self._value
AttributeError: 'NoneType' object has no attribute 'shape'