Invalid operation is performed in: Concat (Forward)

[ikegami-yukino]

When executing ptb/gentext.py based my script using the model trained by train_ptb_nstep.py, then raise following error:

Traceback (most recent call last):
  File "predict.py", line 100, in <module>
    main()
  File "predict.py", line 77, in main
    prob = F.softmax(model.predictor(prev_word))
  File "/work/rnnlm/train.py", line 40, in __call__
    ex = self.embed(F.concat(xs, axis=0))
  File "/work/venv/py3/lib/python3.6/site-packages/chainer/functions/array/concat.py", line 87, in concat
    return Concat(axis=axis)(*xs)
  File "/work/venv/py3/lib/python3.6/site-packages/chainer/function.py", line 189, in __call__
    self._check_data_type_forward(in_data)
  File "/work/venv/py3/lib/python3.6/site-packages/chainer/function.py", line 273, in _check_data_type_forward
    self.check_type_forward(in_type)
  File "/work/venv/py3/lib/python3.6/site-packages/chainer/functions/array/concat.py", line 23, in check_type_forward
    type_check.Variable(self.axis, 'axis'))
  File "/work/venv/py3/lib/python3.6/site-packages/chainer/utils/type_check.py", line 487, in expect
    expr.expect()
  File "/work/venv/py3/lib/python3.6/site-packages/chainer/utils/type_check.py", line 449, in expect
    '{0} {1} {2}'.format(left, self.inv, right))
chainer.utils.type_check.InvalidType: 
Invalid operation is performed in: Concat (Forward)

Expect: in_types[0].ndim > axis
Actual: 0 <= 0