Error using K.round() in custom loss function

In my custom loss function I’m trying to use K.round(y_pred) as part of my calculation. This gives me a ValueError: None values not supported. When I remove the K.round() it all works perfectly. Does anyone know what the problem might be? Thank you very much.

Here is the full error log:

Traceback (most recent call last):
  File "explore_network_structure.py", line 141, in <module>
    history = model.fit(x_train, y_train, nb_epoch=2000, batch_size=batch_size, shuffle=True, verbose=1, class_weight=[class_weight_A, class_weight_B], sample_weight=train_weight, callbacks=[csv_logger, early_stopping])
  File "/usr/local/lib/python2.7/dist-packages/keras/models.py", line 620, in fit
    sample_weight=sample_weight)
  File "/usr/local/lib/python2.7/dist-packages/keras/engine/training.py", line 1079, in fit
    self._make_train_function()
  File "/usr/local/lib/python2.7/dist-packages/keras/engine/training.py", line 696, in _make_train_function
    self.total_loss)
  File "/usr/local/lib/python2.7/dist-packages/keras/optimizers.py", line 154, in get_updates
    v = self.momentum * m - lr * g  # velocity
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/math_ops.py", line 750, in binary_op_wrapper
    y = ops.convert_to_tensor(y, dtype=x.dtype.base_dtype, name="y")
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/ops.py", line 657, in convert_to_tensor
    ret = conversion_func(value, dtype=dtype, name=name, as_ref=as_ref)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/constant_op.py", line 180, in _constant_tensor_conversion_function
    return constant(v, dtype=dtype, name=name)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/constant_op.py", line 163, in constant
    tensor_util.make_tensor_proto(value, dtype=dtype, shape=shape))
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/tensor_util.py", line 346, in make_tensor_proto
    raise ValueError("None values not supported.")
ValueError: None values not supported.

[Dref360]

@yushuinanrong why up? K.round is not differentiable, you cannot use it in a loss function.

[oxrider]

Here is my custom loss:

def func_custom_loss(y_true, y_pred):
    return K.mean(y_pred[:,0], axis=-1)

Currently it is working fine but if I replace y_pred[:,0] with K.round(y_pred[:,0]) it will spit the above error.