title: Keras misinterprets training data shape
body: <p>My training data has the form (?,15) where ? is a variable length.</p>
  
  <p>When creating my model I specify this:</p>
  
  <pre><code>inp = Input(shape=(None,15))
  conv = Conv1D(32,3,padding='same',activation='relu')(inp)
  ...
  </code></pre>
  
  <p>My training data has the shape (35730,?,15).</p>
  
  <p>Checking this in python I get:  </p>
  
  <pre><code>X.shape
  </code></pre>
  
  <p>Outputs: (35730,)</p>
  
  <pre><code>X[0].shape
  </code></pre>
  
  <p>Outputs: (513, 15)</p>
  
  <p>When I try to fit my model on my training data I get the ValueError:  </p>
  
  <pre><code>Error when checking input: expected input_1 to have 3 dimensions, but got array with shape (35730, 1)
  </code></pre>
  
  <p>I can only train my model by using model.train_on_batch() on a single sample.</p>
  
  <p>How can I solve this? It seems like keras thinks the shape of my input data is (35730, 1) when it actually is (35730, ?, 15)</p>
  
  <p>Is this a bug in keras or did I do something wrong?</p>
  
  <p>I am using the tensorflow backend if that matters. This is keras 2</p>