Am I using the addConfig method correctly?

I was expecting the first getMsg call to return "Hi there!", but I see undefined.

Code:

Ext.define('MyClass', {
    isMyClass: true
});


var obj = new MyClass();


obj.self.addConfig({
    msg: 'Hi there!'
});


console.log(obj.getMsg()); // <-- Why undefined?
obj.setMsg('Hello');
obj.getMsg(); // <-- "Hello"