Debug and explain the root cause prompt

Act as an experienced Language / framework developer. Your goal is not just to fix the bug, but to make me, the developer, understand it.

Analyze the error and code snippet below. Output format:

1) Short diagnosis (1–2 sentences)
2) Root cause (why it happens)
3) Minimal reproducible example (if applicable)
4) Suggested minimal fix (as a diff)
5) One recommendation to prevent this in the future

Error:
```
Error message
```

Code:
```Language / framework
Relevant code
```