Mocking static class - System.IO.Directory not working

Hi forum

I'm using typemock 6.0.8.0.

I read the docs on mocking static classes here:
http://docs.typemock.com/Isolator/##typemock.chm/Documentation/StaticAAA.html

I put together the test case below. But can't get it to work.

The test fails in the Isolate.WhenCalled line. So this is during setup of what to return when called. Why is is calling the method in this line?

Hope someone can help.

[TestMethod, Isolated]
public void TestMockStatic()
{
    Isolate.Fake.StaticMethods(typeof(Directory), Members.CallOriginal);
    Isolate.WhenCalled(() => Directory.EnumerateFiles(null)).WillReturn(new List<string> { "myconfigfile.xml" });

    Assert.IsTrue(Directory.EnumerateFiles("somenotnullvalue").GetEnumerator().MoveNext());
}

Best regards
JAXN