title: Error while defining my Keras model: " AttributeError: 'NoneType' object has no attribute '_inbound_nodes' "
body: <p>I'm trying to build a keras model, the problem appeared after adding lines to perform zero-padding then merge 2 layers. The code is as follows:</p>
  
  <pre><code>import keras
  from keras.layers import *
  from keras.models import Model
  from keras.activations import softmax
  import keras.backend as K
  
  query = Input(name='query', shape=(10,))
  doc = Input(name='doc', shape=(100,))
  embedding = Embedding(1125, 50, trainable=True)
  q_embed = embedding(query)
  d_embed = embedding(doc)
  q_w = Dense(1, use_bias=False)(q_embed)
  q_w = Lambda(lambda x: softmax(x, axis=1), output_shape=(10,))(q_w)
  q_w_layer = Lambda(lambda x: K.repeat_elements(q_w, rep=50, axis=2))(q_w)
  q_embed = Multiply()([q_w_layer, q_embed])
  cross = Dot(axes=[2, 2], normalize=False)([q_embed, d_embed])
  cross = Permute((2, 1))(cross)
  contxt = Conv1D(30, 5, strides=5, activation='relu', name="conv")(cross)
  contxt = BatchNormalization()(contxt)
  contxt = Dropout(0.2)(contxt)
  attention = Dense(1, use_bias=False)(contxt)
  attention = Activation('softmax')(attention)
  contxt = Multiply()([contxt, attention])
  important_context = MaxPooling1D(pool_size=2, strides=2)
  contxt = important_context(contxt)
  word_level = Permute((2, 1))(cross)
  
  # ############ This is the part that caused the problem
  
  word_level_padd = K.reshape(ZeroPadding1D((0, contxt.shape[2] - word_level.shape[2]))(K.reshape(word_level,
                                  (-1, 
                                   word_level.shape[2], 
                                   word_level.shape[1]
                                  ))),
                             (-1, word_level.shape[1], contxt.shape[2])
                             ) if word_level.shape[-1] &lt; contxt.shape[-1] else word_level
  contxt_padded = K.reshape(ZeroPadding1D((0, word_level.shape[2] -contxt.shape[2]))(K.reshape(contxt,
                              (-1, 
                               contxt.shape[2], 
                               contxt.shape[1]
                             ))), 
                           (-1, contxt.shape[1], word_level.shape[2])
                           ) if contxt.shape[-1] &lt; word_level.shape[-1] else contxt
  contxt = Concatenate(axis=1, name="merge_levels")([word_level_padd, contxt_padded])
  
  # This is the part that caused the problem #############
  
  lstm_units = int(contxt.shape[1])
  contxt = Bidirectional(LSTM(lstm_units, return_sequences=False))(contxt)
  contxt = BatchNormalization()(contxt)
  contxt = Dropout(0.2)(contxt)
  out_ = Dense(1)(contxt)
  model = Model(inputs=[query, doc], outputs=out_)
  </code></pre>
  
  <p>Here is the error message:</p>
  
  <blockquote>
    <p>Traceback (most recent call last):   File
    "/usr/local/lib/python3.5/dist-packages/IPython/core/interactiveshell.py",
    line 2910, in run_code
        exec(code_obj, self.user_global_ns, self.user_ns)   File "", line 1, in 
        model = Model(inputs=[query, doc], outputs=contxt)   File "/usr/local/lib/python3.5/dist-packages/keras/legacy/interfaces.py",
    line 91, in wrapper
        return func(*args, **kwargs)   File "/usr/local/lib/python3.5/dist-packages/keras/engine/network.py", line
    93, in <strong>init</strong>
        self._init_graph_network(*args, **kwargs)   File "/usr/local/lib/python3.5/dist-packages/keras/engine/network.py", line
    237, in _init_graph_network
        self.inputs, self.outputs)   File "/usr/local/lib/python3.5/dist-packages/keras/engine/network.py", line
    1353, in _map_graph_network
        tensor_index=tensor_index)   File "/usr/local/lib/python3.5/dist-packages/keras/engine/network.py", line
    1340, in build_map
        node_index, tensor_index)   File "/usr/local/lib/python3.5/dist-packages/keras/engine/network.py", line
    1340, in build_map
        node_index, tensor_index)   File "/usr/local/lib/python3.5/dist-packages/keras/engine/network.py", line
    1340, in build_map
        node_index, tensor_index)   File "/usr/local/lib/python3.5/dist-packages/keras/engine/network.py", line
    1340, in build_map
        node_index, tensor_index)   File "/usr/local/lib/python3.5/dist-packages/keras/engine/network.py", line
    1312, in build_map
        node = layer._inbound_nodes[node_index] AttributeError: 'NoneType' object has no attribute '_inbound_nodes'</p>
  </blockquote>