Instantiate nested static class using Class.forName

I have a nested static class like:

package a.b
public class TopClass {

    public static class InnerClass {
    }
}

I want to instantiate with Class.forName() but it raises a ClassNotFoundException .

Class.forName("a.b.TopClass"); // Works fine.
Class.forName("a.b.TopClass.InnerClass"); // raises exception

TopClass.InnerClass instance = new TopClass.InnerClass(); // works fine

What is wrong in my code?

Udo.