Why is it thrown java.lang.InstantiationException?

Look at following code:

public class Outer {
    public static void main(String[] args) throws Exception {
        new Outer().greetWorld();
    }

    private void greetWorld() throws Exception {
        System.out.println(Inner.class.newInstance());
    }

    public class Inner {
        public Inner () {}
        public String toString(){
            return "HelloWorld";
        }
    }
}

Why is it thrown java.lang.InstantiationException ?
After all, nested class Inner has nully constructor. Can someone explain it?