4.2.1 beta fails in mocking URI Property

This following test code fails:

public class StdMessage
{
  public virtual Uri Type
  {
    get
    {
      return new Uri("http://Original.uri/bla");
    }
  }
}

[TestMethod]
[VerifyMocks]
public void TypeMockTest()
{
  StdMessage msg = new StdMessage();
  Uri uriRet = new Uri("http://mocked.uri/bla");
  using (RecordExpectations recorder = RecorderManager.StartRecording())
  {
     // CAUTION: ALL calls here are mocked!!!
     Uri dummy = msg.Type;
     recorder.Return(uriRet);
  }
  Uri value = msg.Type;
}

typemock throws the following exception:

Test method UnitTests.TypeMock.TypeMockTest threw exception: TypeMock.TypeMockException:
*** Cannot return a value for StdMessage.get_Type() because no value was set. use recorder.Return().
*** Note: Cannot mock types from mscorlib assembly..

clearly, i have called recoder.Return()...

if i change the property type from Uri to anything else (that i tried...) it works.
what's wrong with Uri ?