Exception by .WillReturn(XmlDocument)

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.MethodAccessException: System.Xml.XmlNode..ctor().

BetsyWebserviceInterface fake = Isolate.Fake.Instance<BetsyWebserviceInterface>();

antwoord = new XmlDocument();
antwoord.LoadXml("<Polis><Polisnummer>12</Polisnummer></Polis>");            

Isolate.Swap.NextInstance<BetsyWebserviceInterface>().With(fake);
Isolate.WhenCalled(() => fake.GetTabel("T5339", "08")).WillReturn(antwoord);