How to get access to parent field in Java?

I have a class

public class MyException extends RuntimeException {
    public MyException(String s) {
        super(s);
    }
}

And method

void fun() throws NoSuchFieldException, IllegalAccessException {
        Exception ex = new MyException("For fun!");
        Class<? extends Exception> cl = ex.getClass();
        Field msg = cl.getField("detailMessage");
        msg.setAccessible(true);
        msg.set(ex, "Yes!");
        msg.setAccessible(false);   
        System.out.println(ex.getMessage());    
    }

detailMessage is a field in Throwable class. I want to get a message, which has been setted in the child class. How can I do it?

Currently, I'm getting an error:

Exception in thread "main" java.lang.NoSuchFieldException: detailMessage
    at java.lang.Class.getField(Class.java:1556)
    at Test.fun(Test.java:12)
    at Test.main(Test.java:5)