Hi Andres! This happened to some learners before and the issue was on the Coursera lab environment. You might need to simply refresh your notebook and copy your solutions to the new one. Please see this thread for reference: C2W1 Assignment - grader output showed error - #4 by chris.favila . Please let me know if that is indeed the issue so I can tell Coursera that the bug is still there. This does not happen to everyone and so far, I could not replicate it. Thank you!