Tensorflow TypeError: Fetch argument None has invalid type <type 'NoneType'>?

Question

I'm building an RNN loosely based on the TensorFlow tutorial.

The relevant parts of my model are as follows:

input_sequence = tf.placeholder(tf.float32, [BATCH_SIZE, TIME_STEPS, PIXEL_COUNT + AUX_INPUTS]) output_actual = tf.placeholder(tf.float32, [BATCH_SIZE, OUTPUT_SIZE]) lstm_cell = tf.nn.rnn_cell.BasicLSTMCell(CELL_SIZE, state_is_tuple=False) stacked_lstm = tf.nn.rnn_cell.MultiRNNCell([lstm_cell] * CELL_LAYERS, state_is_tuple=False) initial_state = state = stacked_lstm.zero_state(BATCH_SIZE, tf.float32) outputs = [] with tf.variable_scope("LSTM"): for step in xrange(TIME_STEPS): if step > 0: tf.get_variable_scope().reuse_variables() cell_output, state = stacked_lstm(input_sequence[:, step, :], state) outputs.append(cell_output) final_state = state

And the feeding:

cross_entropy = tf.reduce_mean(-tf.reduce_sum(output_actual * tf.log(prediction), reduction_indices=[1])) train_step = tf.train.AdamOptimizer(learning_rate=LEARNING_RATE).minimize(cross_entropy) correct_prediction = tf.equal(tf.argmax(prediction, 1), tf.argmax(output_actual, 1)) accuracy = tf.reduce_mean(tf.cast(correct_prediction, tf.float32)) with tf.Session() as sess: sess.run(tf.initialize_all_variables()) numpy_state = initial_state.eval() for i in xrange(1, ITERATIONS): batch = DI.next_batch() print i, type(batch[0]), np.array(batch[1]).shape, numpy_state.shape if i % LOG_STEP == 0: train_accuracy = accuracy.eval(feed_dict={ initial_state: numpy_state, input_sequence: batch[0], output_actual: batch[1] }) print "Iteration " + str(i) + " Training Accuracy " + str(train_accuracy) numpy_state, train_step = sess.run([final_state, train_step], feed_dict={ initial_state: numpy_state, input_sequence: batch[0], output_actual: batch[1] })

When I run this, I get the following error:

Traceback (most recent call last): File "/home/agupta/Documents/Projects/Image-Recognition-with-LSTM/RNN/feature_tracking/model.py", line 109, in <module> output_actual: batch[1] File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 698, in run run_metadata_ptr) File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 838, in _run fetch_handler = _FetchHandler(self._graph, fetches) File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 355, in __init__ self._fetch_mapper = _FetchMapper.for_fetch(fetches) File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 181, in for_fetch return _ListFetchMapper(fetch) File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 288, in __init__ self._mappers = [_FetchMapper.for_fetch(fetch) for fetch in fetches] File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/client/session.py", line 178, in for_fetch (fetch, type(fetch))) TypeError: Fetch argument None has invalid type <type 'NoneType'>

Perhaps the weirdest part is that this error gets thrown the second iteration, and the first works completely fine. I'm ripping my hair trying to fix this, so any help would be greatly appreciated.

Answer 1

Here on the second iteration, the train_step variable is none because you are re-assigning the train_step variable to the second element of the sess.run result(which is none). Thus, the second iteration is throwing an error. You can simply cover up this problem by discarding the second element of the result.

for i in xrange(1, ITERATIONS):  # Discard the second element of the result. 

numpy_state, _ = sess.run([final_state, train_step], feed_dict={

initial_state: numpy_state, 

input_sequence: batch[0], 

output_actual: batch[1] })

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