Grader error in assignment

My guess is that this means something is damaged about your notebook. Did you use any external tools to edit the notebook? Or did you do everything only on the course website?

One thing to try would be to get a clean copy of the notebook and then carefully “copy/paste” over just your completed code from the “YOUR CODE HERE” segments. There is a procedure documented on the DLS FAQ Thread about how to get a clean copy that is generic and will work for NLP as well.