Faked Behavior Doesn't Change With Subsequent WhenCalled()

public interface IValueSource {
  int Value {
    get;
  }
}


public class ClassUnderTest {
  public int ReturnValue(IValueSource provider) {
    return provider.Value;
  }
}



[Test]
public void TestChangingBehavior() {
    ClassUnderTest obj = new ClassUnderTest();
    IValueSource source = Isolate.Fake.Instance<IValueSource>();
    Isolate.WhenCalled(() => source.Value).WillReturn(1);
    Assert.AreEqual(1, obj.ReturnValue(source));
    Isolate.WhenCalled(() => source.Value).WillReturn(2);
    Assert.AreEqual(2, obj.ReturnValue(source)); //this fails because the returned value is still 1.
}