multiprocessing pool.map error

[kelva]

Hi, professor He, Thanks for sharing your code ,But when I run the it ,There is a bug (as follow). but when I don't use multiprocessing and just use map ,It's ok .
Traceback (most recent call last):
File "D:/project/theano-BPR-master/run_example.py", line 37, in
bpr.build_model(maxIter, num_thread, batch_size=32)
File "D:\project\theano-BPR-master\MFbpr.py", line 88, in build_model
(hits, ndcgs) = evaluate_model(self, self.test, topK, num_thread)
File "D:\project\theano-BPR-master\evaluate.py", line 35, in evaluate_model
res = pool.map(eval_one_rating,range(num_rating))
File "C:\Python27\lib\multiprocessing\pool.py", line 253, in map
return self.map_async(func, iterable, chunksize).get()
File "C:\Python27\lib\multiprocessing\pool.py", line 572, in get
raise self._value
TypeError: 'NoneType' object has no attribute 'getitem'