Mocking Generic Method with NSubstitute

I have an interface with a number of generic methods. These methods perform operations based on the type of data that is passed in. How do I mock this with NSubstitute? At the moment, I had to resort to using a concrete class instead of a mock since I cannot handle all possible types that the method will be called with.

public interface IInstanceSource
{
    bool CanCreate<T>();
    T Create<T>();
    void Register<T>(Func<T> creator);
}

    public static IInstanceSource GetInstanceSource()
    {
        var _data = new Dictionary<Type, Func<object>>();
        var a = Substitute.For<IInstanceSource>();
        //code below fails since T is not defined. How do I make the code below accept any type?
        a.WhenForAnyArgs(x=>x.Register(Arg.Any<Func<T>>)).Do(x=> { /* todo */});
        a.CanCreate<T>().Returns(x => _data[typeof (T)]);
        return a;
    }

thanks.