Debug Helper

Help me debug this error:

**Error Message:**
```
[PASTE ERROR]
```

**Code causing the error:**
```[LANGUAGE]
[PASTE CODE]
```

**What I expected:** [describe expected behavior]
**What happened:** [describe actual behavior]
**Environment:** [OS, language version, framework]

Please:
1. Explain WHY this error occurs
2. Show the EXACT fix with corrected code
3. Explain how to PREVENT this in the future