title: ValueError None values not supported in _make_train_function (Keras)
body: <p>Trying to fit a model in Keras intialized and compiled as below but get None ValueErrors. What are recommended ways to debug such errors? I am new to Keras.</p>
  
  <p>Can I detect the problem earlier, i.e. in initializing or compiling model?</p>
  
  <pre><code>model = Model((64,64,3))
  opt = keras.optimizers.Adam(lr=0.001, beta_1=0.9, beta_2=0.999, epsilon=None, decay=0.0)
  binLoss = "binary_crossentropy"
  model.compile(optimizer = opt, loss = binLoss, metrics = ["accuracy"])
  
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-15-5b61099068d8&gt; in &lt;module&gt;()
        1 ### START CODE HERE ### (1 line)
  ----&gt; 2 happyModel.fit(x = X_train, y = Y_train, epochs = 100, batch_size = 32)
        3 ### END CODE HERE ###
  
  /opt/conda/lib/python3.6/site-packages/keras/engine/training.py in fit(self, x, y, batch_size, epochs, verbose, callbacks, validation_split, validation_data, shuffle, class_weight, sample_weight, initial_epoch, steps_per_epoch, validation_steps, **kwargs)
     1574         else:
     1575             ins = x + y + sample_weights
  -&gt; 1576         self._make_train_function()
     1577         f = self.train_function
     1578 
  ...
  /opt/conda/lib/python3.6/site-packages/tensorflow/python/framework/tensor_util.py in make_tensor_proto(values, dtype, shape, verify_shape)
      362   else:
      363     if values is None:
  --&gt; 364       raise ValueError("None values not supported.")
      365     # if dtype is provided, forces numpy array to be the type
      366     # provided if possible.
  
  ValueError: None values not supported.
  </code></pre>