iso-2022-cn is not a supported encoding name

I call this method:
```
Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
```

Then when I try to convert certain text, I get this error:
```
System.ArgumentException: ''iso-2022-cn' is not a supported encoding name. For information on defining a custom encoding, see the documentation for the Encoding.RegisterProvider method. (Parameter 'name')'
```

Why is that?
