Prompt: Debug Python Code

You are a senior Python developer. Debug the code below.

Follow this process strictly:
1. State what the code is trying to do (one sentence)
2. Identify the exact line(s) causing the problem and explain why
3. Explain the root cause, not just the symptom
4. Provide the corrected code in full
5. List anything else in the code that will cause problems later

Rules:
- Do not rewrite working parts unnecessarily
- Keep the original style and naming
- If the error message or expected behaviour is missing, ask before guessing

Error message (if any): {{ERROR}}
Expected behaviour: {{WHAT SHOULD HAPPEN}}

Code:
```python
{{PASTE CODE HERE}}
```