chainerで回帰を実装中に発生したエラーについて

発生している問題・エラーメッセージ

chainerで回帰を実装している最中に以下のエラーが発生してしまいました。

ソースコード

python
1import chainer
2import chainer.functions as F
3import chainer.links as L
4import numpy as np
5
6from chainer import optimizers
7
8class MLP(chainer.Chain):
9
10    def __init__(self):
11        super().__init__()
12        with self.init_scope():
13            self.c1 = L.Convolution2D(None, 16, 3, pad=1)
14            self.b1 = L.BatchNormalization(16)
15            self.f1 = L.Linear(None, 64)
16
17    def __call__(self, x, t):
18        y = self.predictor(x)
19        self.loss = F.huber_loss(y, t, 1.0)
20        return self.loss
21
22    def predictor(self, x):
23        x = F.relu(self.b1(self.c1(x)))
24        x = self.f1(x)
25        return x
26            
27
28
29x = np.arange(2 * 5 * 8 * 8).reshape(2, 5, 8, 8).astype("f")
30t = np.arange(2 * 64).reshape(2, 64).astype("f")
31
32model = MLP()
33
34optimizer = optimizers.Adam()
35optimizer.setup(model)
36
37optimizer.update(model, x, t)

エラーメッセージ

Traceback (most recent call last):
  File "c:/Users/ubdc2/Desktop/reversi_rl/reversi_main.py", line 643, in <module>
    optimizer.update(model, x, t)
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Anaconda3_64\lib\site-packages\chainer\optimizer.py", line 685, in update
    loss.backward(loss_scale=self._loss_scale)
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Anaconda3_64\lib\site-packages\chainer\variable.py", line 963, in backward
    self._backward_main(retain_grad, loss_scale)
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Anaconda3_64\lib\site-packages\chainer\variable.py", line 1040, in _backward_main
    func, target_input_indexes, out_grad, in_grad)
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Anaconda3_64\lib\site-packages\chainer\_backprop_utils.py", line 106, in backprop_step
    target_input_indexes, grad_outputs)
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Anaconda3_64\lib\site-packages\chainer\functions\loss\huber_loss.py", line 57, in backward
    gy = chainer.functions.expand_dims(gy, 1)
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Anaconda3_64\lib\site-packages\chainer\functions\array\expand_dims.py", line 73, in expand_dims
    y, = ExpandDims(axis).apply((x,))
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Anaconda3_64\lib\site-packages\chainer\function_node.py", line 245, in apply
    self._check_data_type_forward(in_data)
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Anaconda3_64\lib\site-packages\chainer\function_node.py", line 322, in _check_data_type_forward
    self.check_type_forward(in_type)
  File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Anaconda3_64\lib\site-packages\chainer\functions\array\expand_dims.py", line 18, in check_type_forward
    type_check.expect(x_type.ndim >= self.axis)
AttributeError: 'NoneType' object has no attribute 'ndim'

解決策を探しています。
よろしくお願いいたします。