Debug This Code Expert

I have a bug in my [LANGUAGE] code. Help me debug it.

Code:
```[LANGUAGE]
[YOUR BUGGY CODE]
```

Expected behavior: [WHAT SHOULD HAPPEN]
Actual behavior: [WHAT IS HAPPENING]
Error message (if any): [ERROR]

Please:
1. Identify the root cause
2. Explain why the bug occurs
3. Provide the fixed code
4. Suggest how to prevent similar bugs
5. Add any relevant tests