Course 4 Week 4 Assignment 2 - all tests passed but 80% mark

Course Q&A Deep Learning Specialization Convolutional Neural Networks
1

All my code sells when I ran them passsed all tests, but the grader came back with this output and gave me 80% mark. What’s wrong?

Code Cell UNQ_C1: Function ‘compute_content_cost’ is correct.
Code Cell UNQ_C2: Function ‘gram_matrix’ is correct.
Code Cell UNQ_C3: Function ‘compute_layer_style_cost’ is correct.
Code Cell UNQ_C4: Function ‘total_cost’ is correct.
Code Cell UNQ_C5: Unexpected error (TypeError(‘in user code:\n\n /tmp/student_solution_cells/cell_26.py:56 train_step *\n grad = tape.gradient(J, generated_image)\n /usr/local/lib/python3.7/site-packages/tensorflow/python/eager/backprop.py:1034 gradient **\n if not backprop_util.IsTrainable(t):\n /usr/local/lib/python3.7/site-packages/tensorflow/python/eager/backprop_util.py:30 IsTrainable\n dtype = dtypes.as_dtype(dtype)\n /usr/local/lib/python3.7/site-packages/tensorflow/python/framework/dtypes.py:650 as_dtype\n (type_value,))\n\n TypeError: Cannot convert value None to a TensorFlow DType.\n’)) occurred during function check. We expected function train_step to return train_step test 1 failed. Please check that this function is defined properly.
If you see many functions being marked as incorrect, try to trace back your steps & identify if there is an incorrect function that is being used in other steps.
This dependency may be the cause of the errors.

2

There appears to be an error in your UNQ_C5 cell.