getting a runtime error

The problem is that this code runs fine on my Windows PC and also runs fine on my AWS Linux server. So I get the feeling this is a configuration "thing". From the Docs:

Extending Core Class

If all you need to do is add some functionality to an existing library - perhaps add a method or two - then it’s overkill to replace the entire library with your version. In this case it’s better to simply extend the class. Extending a class is nearly identical to replacing a class with a couple exceptions:

• The class declaration must extend the parent class.
  
  
• Your new class name and filename must be prefixed with MY_ (this item is configurable. See below.).
  

For example, to extend the native Input class you’ll create a file named application/core/MY_Input.php, and declare your class with:

Looking in config.php I have $config['subclass_prefix'] = 'MY_';

So why would the code be looking for loader.php instead of MY_loader.php? I didn't load composer on the Ubuntu PC. Do I need to?

proof that an old dog can learn new tricks