title: Error: 'NoneType' object has no attribute '_inbound_nodes'
body: <p>[enter image description here][1]I am trying to make a parallel ANN network.
  I plan to :</p>
  
  <ul>
  <li>input a 120X120 image.</li>
  <li>disintegrate it into 9 40x40 images.</li>
  <li>Run Convolutional Net.</li>
  <li>Merge output in same pattern.</li>
  <li>Run another conv-net on merged layer.</li>
  </ul>
  
  <p></p>
  
  <pre><code>def conv_net(): 
      input_shape = [120,120,1]
      inp=Input(shape=input_shape)
      print(type(inp))
      print(inp.shape)
      row_layers  = []
      col_layers  = []
  
      # fn = lambda x: self.conv(x)
      for i in range(0, 120, 40):
          row_layers = []
  
          for j in range(0, 120, 40):
              # out = (self.conv(inp[:,i:i+39,j:j+39]))
              inputs = inp[:, i:i + 40, j:j + 40]
  
              x = Dense(64, activation='relu')(inputs)
              out = Dense(64, activation='relu')(x)
              print(out.shape)
              row_layers.append(out)
          col_layers.append(keras.layers.concatenate(row_layers, axis=2))
          print((len(col_layers)))
      merged = keras.layers.concatenate(col_layers, axis=1)
      print(merged.shape)
      con = Conv2D(1, kernel_size=5, strides=2, padding='same', activation='relu')(merged)
      print(con.shape)
      output = Flatten()(con)
      output = Dense(1)(output)
      print(output.shape)
  
      model = Model(inputs=inp, outputs=output)
      # plot_model(model,to_file='model.png')
      return model
  </code></pre>
  
  <p>I am getting an error <code>NoneType</code> object has no attribute <code>_inbound_nodes</code>.</p>
  
  <p>I debug a little. And the error is becuase of this line.</p>
  
  <pre><code>inputs = inp[:,i:i+40,j:j+40]
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:/Users/Todd Letcher/machine_learning_examples/unsupervised_class3/slicing_img.py", line 83, in &lt;module&gt;
      conv_net()
    File "C:/Users/Todd Letcher/machine_learning_examples/unsupervised_class3/slicing_img.py", line 80, in conv_net
      model = Model(inputs=inp, outputs = output)
    File "C:\Users\Todd Letcher\AppData\Local\Programs\Python\Python36\lib\site-packages\keras\legacy\interfaces.py", line 91, in wrapper
      return func(*args, **kwargs)
    File "C:\Users\Todd Letcher\AppData\Local\Programs\Python\Python36\lib\site-packages\keras\engine\network.py", line 91, in __init__
      self._init_graph_network(*args, **kwargs)
    File "C:\Users\Todd Letcher\AppData\Local\Programs\Python\Python36\lib\site-packages\keras\engine\network.py", line 235, in _init_graph_network
      self.inputs, self.outputs)
    File "C:\Users\Todd Letcher\AppData\Local\Programs\Python\Python36\lib\site-packages\keras\engine\network.py", line 1406, in _map_graph_network
      tensor_index=tensor_index)
    File "C:\Users\Todd Letcher\AppData\Local\Programs\Python\Python36\lib\site-packages\keras\engine\network.py", line 1393, in build_map
      node_index, tensor_index)
    File "C:\Users\Todd Letcher\AppData\Local\Programs\Python\Python36\lib\site-packages\keras\engine\network.py", line 1393, in build_map
      node_index, tensor_index)
    File "C:\Users\Todd Letcher\AppData\Local\Programs\Python\Python36\lib\site-packages\keras\engine\network.py", line 1393, in build_map
      node_index, tensor_index)
    File "C:\Users\Todd Letcher\AppData\Local\Programs\Python\Python36\lib\site-packages\keras\engine\network.py", line 1365, in build_map
      node = layer._inbound_nodes[node_index]
  AttributeError: 'NoneType' object has no attribute '_inbound_nodes'
  </code></pre>
  
  <p>Help appreciated. Thank you</p>
  
  <p>P.S.:I removed the slicing line <code>inp[:,i:i+39,j:j+39]</code> and it runs ok.</p>
  
  <p>Image shows what I intend to do. The only difference is that I want to split the image into 9 tiles. Here the same image is fed to all the parallel Conv-nets.</p>
  
  <pre><code>[1]: https://i.stack.imgur.com/Z7nt0.png
  </code></pre>