Error: 'this' is only defined in non-static member functions

[A Guy With a Question via Digitalmars-d-learn]

I have an interface where I have a classes embedded in it's scope 
(trying to create something for the classes that implement the 
interface can use for unittesting).

    interface IExample
    {
        // stuff ...
        class Tester
        {
        }
    }

I'm trying to make an instance of it like:

    auto tester = new IExample.Tester();

And I get the above error.