Polymorphism not working for a call from a generic class in C#

It looks like in the following case the polymorphism does not work properly I have the following definitions:

interface BaseInterface{}
interface NewInterface:BaseInterface{}
class NewClass:NewInterface{}

class GenericClass<T> where T:BaseInterface
{
    public string WhoIAm(T anObject)
    {
        return TestPolymorphism.CheckInterface(anObject);
    }
}

class ImplementedClass:GenericClass<NewInterface>{}

class TestPolymorphism
{
    public static string CheckInterface(BaseInterface anInterface)
    {
        return "BaseInterface";
    }

    public static string CheckInterface(NewInterface anInterface)
    {
        return "NewInterface";
    }
}

Then when I call :

NewClass nc = new NewClass();
ImplementedClass impClass = new ImplementedClass();
Console.WriteLine("The result is " + impClass.WhoIAm(nc));

I have "The result is BaseInterface"

I was expecting to have "The result is NewInterface" as nc implement BaseClass and NewClass
What would be the best way to get "NewClass" as the result ?

Thanks