This code doesn't throw an expected NestedCallException

public class A
{
  public AFormatFileWriter FileWriter { get; private set; }

  public A()
  {
     FileWriter = new AFormatFileWriter();
  }

  public void Method()
  {
     FileWriter.WriteToFile();
  }

  public class AFormatFileWriter
  {
     public void WriteToFile()
     {
     }
  }
}

[TestMethod]
public void Test()
{
  Isolate.Swap.AllInstances<A>().With(
     Isolate.Fake.Instance<A>());
  A a = Isolate.Fake.Instance<A>(Members.CallOriginal);
  //
  a.Method();
  //
  Isolate.Verify.WasCalledWithExactArguments(() => 
     a.FileWriter.WriteToFile());
}