TypeError: Expected Operation, Variable, or Tensor, got 0

[bewithme]

Traceback (most recent call last):
File "/Users/wenfengxu/idea-workspace/CRNN-Keras/training.py", line 11, in
model = get_Model(training=True)
File "/Users/wenfengxu/idea-workspace/CRNN-Keras/Model.py", line 62, in get_Model
lstm_1 = LSTM(256, return_sequences=True, kernel_initializer='he_normal', name='lstm1')(inner) # (None, 32, 512)
File "/Users/wenfengxu/opt/anaconda3/envs/CRNN-Keras/lib/python3.5/site-packages/keras/layers/recurrent.py", line 536, in call
return super(RNN, self).call(inputs, **kwargs)
File "/Users/wenfengxu/opt/anaconda3/envs/CRNN-Keras/lib/python3.5/site-packages/keras/engine/base_layer.py", line 489, in call
output = self.call(inputs, **kwargs)
File "/Users/wenfengxu/opt/anaconda3/envs/CRNN-Keras/lib/python3.5/site-packages/keras/layers/recurrent.py", line 2242, in call
initial_state=initial_state)
File "/Users/wenfengxu/opt/anaconda3/envs/CRNN-Keras/lib/python3.5/site-packages/keras/layers/recurrent.py", line 677, in call
input_length=timesteps)
File "/Users/wenfengxu/opt/anaconda3/envs/CRNN-Keras/lib/python3.5/site-packages/keras/backend/tensorflow_backend.py", line 3105, in rnn
targets=[last_output])
File "/Users/wenfengxu/opt/anaconda3/envs/CRNN-Keras/lib/python3.5/site-packages/tensorflow_core/python/keras/utils/tf_utils.py", line 134, in get_reachable_from_inputs
raise TypeError('Expected Operation, Variable, or Tensor, got ' + str(x))
TypeError: Expected Operation, Variable, or Tensor, got 0

Process finished with exit code 1