C# interface inheritance

Given:

public interface IA
{
    void TestMethod();
}

public interface IB : IA
{
}

Why:

typeof(IB).GetMethods().Count() == 0;

?

Just to be clear:

public class A
{
    public void TestMethod()
    {
    }
}

public class B : A
{
}

typeof(B).GetMethods().Count();

does work (it returns 5);

As a bonus:

typeof(IB).BaseType == null