python-2.7 - Chainer's Link becomes "NoneType"

Question

I'm trying to construct an LSTM network with multi GPUs using Chainer (v4.0.0b1). As in the following code.

import numpy as np
import chainer
from chainer import optimizers, Chain, training, iterators, serializers, cuda, Variable
import chainer.functions as F
import chainer.links as L

...

class Network(Chain):
    def __init__(self):
        super(Network, self).__init__()
        with self.init_scope():
            ...
            self.fc1  = L.Liner(3000, 1000).to_gpu(1)
            self.lstm = L.LSTM(1000, 1000).to_gpu(1)
            self.fc2  = L.Liner(1000, 3000).to_gpu(1)
            ...

    def __call__(self, x, t):
        ...

...

However, the LSTM link becomes "NoneType". As in the following error in call.

TypeError: 'NoneType' object is not callble

I thought it was strange so I displayed "self.lstm". As a result, 'None' was displayed. For example, fc1 which is "Link" is displayed as follows.

<chainer.links.connection.linear.Linear object at hogehoge>

I found out that "self.lstm" could not be declared as Link in "self.lstm = L.LSTM(1000, 1000).to_gpu(1)". However, I don't know why I can't declare it.

I use Chainer's Docker as the execution environment.

Thank you for answering.