title: ValueError with Concatenate Layer (Keras functional API)
body: <p>After some search here, I still can't find a solution for this. I'm new to Keras, apologies if there is a solution and I actually didn't understand how it was related to my problem.</p>
  
  <p>I am making a small RNN with Keras 2/Functional API, and I have trouble to make the Concatenate Layer work.</p>
  
  <p>Here is my structure :</p>
  
  <pre><code>inputSentence = Input(shape=(30, 91))
  sentenceMatrix = LSTM(91, return_sequences=True, input_shape=(30, 91))(inputSentence)
  
  inputDeletion = Input(shape=(30, 1))
  deletionMatrix = (LSTM(30, return_sequences=True, input_shape=(30, 1)))(inputDeletion)
  
  fusion = Concatenate([sentenceMatrix, deletionMatrix])
  fusion = Dense(122, activation='relu')(fusion)
  fusion = Dense(102, activation='relu')(fusion)
  fusion = Dense(91, activation='sigmoid')(fusion)
  
  F = Model(inputs=[inputSentence, inputDeletion], outputs=fusion)
  </code></pre>
  
  <p>And here is the error:</p>
  
  <pre><code>ValueError: Unexpectedly found an instance of type `&lt;class 'keras.layers.merge.Concatenate'&gt;`. Expected a symbolic tensor instance.
  </code></pre>
  
  <p>Full History if it helps a bit more :</p>
  
  <pre><code>Using TensorFlow backend.
      str(inputs) + '. All inputs to the layer '
  ValueError: Layer dense_1 was called with an input that isn't a symbolic tensor. Received type: &lt;class 'keras.layers.merge.Concatenate'&gt;. Full input: [&lt;keras.layers.merge.Concatenate object at 0x00000000340DC4E0&gt;]. All inputs to the layer should be tensors.
  self.assert_input_compatibility(inputs)
    File "C:\ProgramData\Anaconda3\lib\site-packages\keras\engine\topology.py", line 425, in assert_input_compatibility
  fusion = Dense(122, activation='relu')(fusion)
    File "C:\ProgramData\Anaconda3\lib\site-packages\keras\engine\topology.py", line 552, in __call__
  Traceback (most recent call last):
    File "C:\ProgramData\Anaconda3\lib\site-packages\keras\engine\topology.py", line 419, in assert_input_compatibility
  K.is_keras_tensor(x)
    File "C:\ProgramData\Anaconda3\lib\site-packages\keras\backend\tensorflow_backend.py", line 392, in is_keras_tensor
  raise ValueError('Unexpectedly found an instance of type `' + str(type(x)) + '`. '
  ValueError: Unexpectedly found an instance of type `&lt;class 'keras.layers.merge.Concatenate'&gt;`. Expected a symbolic tensor instance.
  </code></pre>
  
  <p>I'm using Python 3.6, with Spyder 3.1.4, on Windows 7. I upgraded TensorFlow and Keras with pip this morning.</p>
  
  <p>Thank you for any help provided !</p>