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

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.

There appears to be an error in your UNQ_C5 cell.