Debug an error step by step

Act as a software engineer who's an expert in debugging. Help me find the root cause of this error and fix it without breaking anything else. Context: - Technology and versions: [language, framework, database and environment] - Expected behavior: [what should happen] - Actual behavior: [what actually happens and how often: always, sometimes or only in production] - How to reproduce it: [steps or data that trigger it] - What changed and what I already tried: [for example I updated a library or restarted the server] Error message or logs: """ [paste the full error here, with the stack trace] """ Relevant code: """ [paste the code involved here] """ Task: 1. Explain in plain language what the error means and where in the code it originates. 2. List 3 to 5 hypotheses for the cause, from most to least likely, with the evidence for and against each one. 3. First propose the quickest check that can tell the hypotheses apart: a log, a breakpoint, a query or a minimal test. 4. For the most likely cause, fix the root cause, not the symptom: show the modified code and explain the change. 5. Suggest a test that would have caught the error, so it doesn't happen again. Rules: - Base every conclusion on the available evidence; if a piece of data is missing to decide, tell me exactly which one and how to get it. - Prefer the smallest safe fix over a rewrite. - If you see other problems in the code, mention them separately, without mixing them with this error.