W1A3: nbgrader-part fails 60/100 with "Tests failed on 1 cell(s)!"

Course Q&A Deep Learning Specialization Improving Deep Neural Networks: Hyperparameter tun
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Here’s another thread that describes an error on that assignment that produces that behavior. Definitely worth a look. The overall point is that you don’t need to do any type coercion on the difference value that you are computing.