title: Keras: ValueError: Input 0 is incompatible layer issues
body: <p>I am using Keras with Tensorflow as backends and get incompatible errors:</p>
  
  <pre><code>model = Sequential()
  model.add(LSTM(64, input_dim = 1))
  model.add(Dropout(0.2))
  model.add(LSTM(16))
  </code></pre>
  
  <p>The following error shows:</p>
  
  <pre><code>Traceback (most recent call last):
    File "train_lstm_model.py", line 36, in &lt;module&gt;
      model.add(LSTM(16))
    File "/home/***/anaconda2/lib/python2.7/site-packages/keras/models.py", line 332, in add
      output_tensor = layer(self.outputs[0])
    File "/home/***/anaconda2/lib/python2.7/site-packages/keras/engine/topology.py", line 529, in __call__
      self.assert_input_compatibility(x)
    File "/home/***/anaconda2/lib/python2.7/site-packages/keras/engine/topology.py", line 469, in assert_input_compatibility
      str(K.ndim(x)))
  ValueError: Input 0 is incompatible with layer lstm_2: expected ndim=3, found ndim=2
  </code></pre>
  
  <p>How can I fix this problem?</p>
  
  <p>Keras version: 1.2.2
  Tensorflow version: 0.12</p>