title: Object has no attribute 'inbound_nodes'
body: <p>I am trying to define a model an compile it, but I am for some reason not able to compile or define this model... </p>
  
  <pre><code>def fws():
      filter_size = 8
      pooling_size = 6
      stride_step = 2
      J = 80
      splits = 33
      total_frames_with_deltas = 45
      pool_splits = ((splits - pooling_size)+1)/2
      print "pool_splits" + str(pool_splits)
      print "Printing shapes"
  
      list_of_input = [Input(shape=(8,3)) for i in range(splits*total_frames_with_deltas)]
      output_convolution = []
  
      for steps in range(total_frames_with_deltas):
          conv = Conv1D(filters = J, kernel_size = 8)
          column = 0
          skip = 45
          conv_output = []
          for _ in range(splits):
              conv_output.append(conv(list_of_input[(column*skip)+steps]))
              column = column + 1
          output_convolution.append((conv_output))
  
      print len(output_convolution)
      print len(output_convolution[0])
  
      out = 0
      output_conv = []
  
      for row in range(splits):
          for column in range(total_frames_with_deltas):
              #print row
              #print column
              out = out + output_convolution[column][row]
          output_conv.append(out)
  
      output_con = Concatenate()(output_conv)
      output_con = Reshape((splits,-1))(output_con)
  
      pooled = MaxPooling1D(pool_size = pooling_size, strides = stride_step)(output_con)
      print pooled.shape
      #reshape = Reshape((3,-1))(pooled)
  
      #fc
      dense1 = Dense(units = 1000, activation = 'relu',    name = "dense_1")(pooled)
      dense2 = Dense(units = 1000, activation = 'relu',    name = "dense_2")(dense1)
      dense3 = Dense(units = 50 , activation = 'softmax', name = "dense_3")(dense2)
      raw_input("Model definition ok!")
  
      model = Model(inputs = list_of_input , outputs = dense3)
      raw_input("Model definition with input/output")
  
      model.compile(loss="categorical_crossentropy", optimizer='sgd' , metrics = [metrics.categorical_accuracy])
  </code></pre>
  
  <p>This  is the full error message:</p>
  
  <pre><code>  File "keras_cnn_phoneme_original_fit_generator.py", line 231, in &lt;module&gt;
      fws()
    File "keras_cnn_phoneme_original_fit_generator.py", line 212, in fws
      model = Model(inputs = list_of_input , outputs = dense3)
    File "/usr/local/lib/python2.7/dist-packages/keras/legacy/interfaces.py", line 88, in wrapper
      return func(*args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/keras/engine/topology.py", line 1676, in __init__
      build_map_of_graph(x, finished_nodes, nodes_in_progress)
    File "/usr/local/lib/python2.7/dist-packages/keras/engine/topology.py", line 1666, in build_map_of_graph
      layer, node_index, tensor_index)
    File "/usr/local/lib/python2.7/dist-packages/keras/engine/topology.py", line 1666, in build_map_of_graph
      layer, node_index, tensor_index)
    File "/usr/local/lib/python2.7/dist-packages/keras/engine/topology.py", line 1666, in build_map_of_graph
      layer, node_index, tensor_index)
    File "/usr/local/lib/python2.7/dist-packages/keras/engine/topology.py", line 1666, in build_map_of_graph
      layer, node_index, tensor_index)
    File "/usr/local/lib/python2.7/dist-packages/keras/engine/topology.py", line 1666, in build_map_of_graph
      layer, node_index, tensor_index)
    File "/usr/local/lib/python2.7/dist-packages/keras/engine/topology.py", line 1664, in build_map_of_graph
      next_node = layer.inbound_nodes[node_index]
  AttributeError: 'NoneType' object has no attribute 'inbound_nodes'
  </code></pre>
  
  <p>The error seem to occur when define the input and output of the network..
  I am not sure why.. Both the convolution and pooling has been designed to handle the input.. So the error makes no sense to me?</p>