Tensorflow TypeError: Fetch argument None has invalid type ?

Question

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

The relevant parts of my model are as follows:

input_sequence = tf.placeholder(tf.float         


        

Answer 1

You are re-assigning the train_step variable to the second element of the result of sess.run() (which happens to be None). Hence, on the second iteration, train_step is None, which leads to the error.

The fix is fortunately simple:

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]
        })

Answer 2

Another common reason to get this error is if you include the summary fetch operation but have not written any summaries.

Example:

# tf.summary.scalar("loss", loss) # <- uncomment this line and it will work fine
summary_op = tf.summary.merge_all()
sess = tf.Session()
# ...
summary = sess.run([summary_op, ...], feed_dict={...}) # TypeError, summary_op is "None"!

What's extra confusing is that summary_op is not itself None, that's just the error that bubbles up from inside the session's run method.