Faking a SPWebApplication object

Hi,

I'm currently TypeMock Isolator for SharePoint. I've already read the example unit test which mostly address SPSite, SPWeb, SPList and SPListItem classes.

Now I'm digging a little bit deeper, trying to fake other object types.

So I wrote following little class for which I want to do a unit test. The class consists of a constructor which takes the URL of a SharePoint web application and a method which returns the managed paths defined for this web application in a list.

public class Logic
    {
        private SPWebApplication webApp;

        public CourseSiteCreation(string webAppURLString)
        {            
                Uri webAppURL = new Uri(webAppURLString);
                webApp = SPWebApplication.Lookup(webAppURL);            
        }

        public List<string> GetManagedPathsList()
        {
            List<string> retList = new List<string>();
            SPPrefixCollection prefixes = webApp.Prefixes;  

            foreach (SPPrefix prefix in prefixes)
            {
                retList.Add(prefix.Name);
            }
            return retList;
        }

To test this method I want to fake the SPWebApplication webApp within the Logic class so that it will return 3 maneged paths when the GetManagedPathsList method is tested.

My unit test method looks like this:

 [Test]
 [Isolated]
 public void GetManagedPaths_RequestAllManagedPaths_ValidAmountOfPaths()
        {
            SPWebApplication fakeWebApp = Isolate.Fake.Instance<SPWebApplication>
           (Members.ReturnRecursiveFakes);

            Isolate.Swap.NextInstance<SPWebApplication>().With(fakeWebApp);
            SPPrefix fakedPrefix = Isolate.Fake.Instance<SPPrefix>(Members.ReturnRecursiveFakes);             
            Isolate.WhenCalled(() => fakeWebApp.Prefixes).WillReturnCollectionValuesOf(new[]
                             {fakedPrefix, fakedPrefix, fakedPrefix});

            Logic webApp = new Logic ("http://www.dummy.com");

            List<string> list = webApp.GetManagedPathsList();

            Assert.AreEqual(3, list.Count);
        }

When I run this unit test a System.NullReferenceException is thrown at the line starting with "Isolate.WhenCalled(..." .

What am I doing wrong?