npzファイルのpickle化に失敗する

前提・実現したいこと

2D画像から3Dスケルトンモデルを構築するプログラムの構築を行っています。
しかし途中でエラーが起こり、プログラムが止まってしまいました。
3DPose-gun
上記のサイトを参考にしています。

発生している問題・エラーメッセージ

deepstation@deepstation:~/3dpose_gan$ python3 bin/demo.py sample/image.png --lift_model sample/gen_epoch_500.npz --model2d pose_iter_440000.caffemodel --proto2d openpose_pose_coco.prototxt
MODEL: generator, N_OUT: 17, N_UNIT: 1024
Traceback (most recent call last):
  File "/home/deepstation/.virtualenvs/dwango/lib/python3.4/site-packages/numpy/lib/npyio.py", line 440, in load
    return pickle.load(fid, **pickle_kwargs)
_pickle.UnpicklingError: invalid load key, 'v'.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "bin/demo.py", line 199, in <module>
    main(args)
  File "bin/demo.py", line 157, in main
    chainer.serializers.load_npz(args.lift_model, model)
  File "/home/deepstation/.virtualenvs/dwango/lib/python3.4/site-packages/chainer/serializers/npz.py", line 187, in load_npz
    with numpy.load(file) as f:
  File "/home/deepstation/.virtualenvs/dwango/lib/python3.4/site-packages/numpy/lib/npyio.py", line 443, in load
    "Failed to interpret file %s as a pickle" % repr(file))
OSError: Failed to interpret file 'sample/gen_epoch_500.npz' as a pickle

該当のソースコード（エラー付近のみ抜粋）

python
1    try:
2        # Code to distinguish from NumPy binary files and pickles.
3        _ZIP_PREFIX = b'PK\x03\x04'
4        N = len(format.MAGIC_PREFIX)
5        magic = fid.read(N)
6        # If the file size is less than N, we need to make sure not
7        # to seek past the beginning of the file
8        fid.seek(-min(N, len(magic)), 1)  # back-up
9        if magic.startswith(_ZIP_PREFIX):
10            # zip-file (assume .npz)
11            # Transfer file ownership to NpzFile
12            tmp = own_fid
13            own_fid = False
14            return NpzFile(fid, own_fid=tmp, allow_pickle=allow_pickle,
15                           pickle_kwargs=pickle_kwargs)
16        elif magic == format.MAGIC_PREFIX:
17            # .npy file
18            if mmap_mode:
19                return format.open_memmap(file, mode=mmap_mode)
20            else:
21                return format.read_array(fid, allow_pickle=allow_pickle,
22                                         pickle_kwargs=pickle_kwargs)
23        else:
24            # Try a pickle
25            if not allow_pickle:
26                raise ValueError("allow_pickle=False, but file does not contain "
27                                 "non-pickled data")
28            try:
29                return pickle.load(fid, **pickle_kwargs)
30            except Exception:
31                raise IOError(
32                    "Failed to interpret file %s as a pickle" % repr(file))
33    finally:
34        if own_fid:
35            fid.close()
36