Hey @anshi, the graders are functioning properly, please don’t have any doubts about that.
This error is related to your notebook.
I’d recommend SAVING YOUR WORK ON YOUR LOCAL MACHINE or rename the file, and get a fresh copy of the notebook from the “Help” menu on the top right. You might have unknowingly changed some code which you shouldn’t have had in the notebook.
I got this exact error and it turned out the solution was this one by Meir.
In “Cell 3” - i.e. the third executable cell, just under " Check out the some of the unmasked and masked images from the dataset", I had changed the index N of the sample image to 10. This worked in the notebook, but broke the grader.