Fix a Bug

<error_and_stack_trace> {{Error message and stack trace}} </error_and_stack_trace> <code> {{Relevant code}} </code> <what_happened> {{What you were doing}} </what_happened> </context> <task> Using the error, the code and my notes above, find the root cause and give me a fix. A good answer: - names the line that throws and, separately, the line or input where the problem starts, and shows how one leads to the other - accounts for every clue in my notes, including the cases where the error does not happen - fixes the cause rather than only silencing the error - tells me how to confirm the cause in a minute or two before I change an