title: ValueError: The shape of the input to "Flatten" is not fully defined (Neural Networks)
body: <p>I try to using Flatten for my LSTM model, but it doesn't work and getting this error:</p>
  <blockquote>
  <p>ValueError: The shape of the input to &quot;Flatten&quot; is not fully defined (got (None, 450) Make sure to pass a complete &quot;input_shape&quot; or &quot;batch_input_shape&quot; argument to the first layer in your model.</p>
  </blockquote>
  <h3>Model:</h3>
  <pre><code>input_vector_size = kwargs.get('input_vector_size', 128)
  dense_size = kwargs.get('dense_size', 20)
  timesteps = None
  xav_init = tf.contrib.layers.xavier_initializer()
  ##########
  
  model = Sequential()
  model.add(LSTM(int(input_vector_size), input_shape=timesteps,int(input_vector_size)), return_sequences=True))
  model.add(SeqSelfAttention(attention_activation='softmax'))
  model.add(Flatten())
  model.add(Dense(dense_size, activation='sigmoid', kernel_initializer=xav_init))
  model.add(Dense(kwargs.get('label_size', 2), activation='sigmoid'))
  </code></pre>
  <p>How do I solve this problem?</p>