WillReturn(null) is throwing a nullreferenceexception

I have somce code that worked in a previous version of isolator for .net. Recently upgraded to 6.1.2.0, and now, the .WillReturn method is throwing a NullReferenceException when it trying to use reflection to invoke a method.

Below I have included a portion of the test, and the first call to WillReturn() is throwing the exception.

Any ideas?

ObjectState setManager = new ObjectState(identity);
setManager.SetField("mManager", this.Manager);

System.Data.Common.DbDataReader fakeReader = Isolate.Fake.Instance<System>();

// THIS THROWS THE EXCEPTION
Isolate.WhenCalled(() => this.Manager.RunQuery((Tlr.Data.SqlQueryInformation)null)).WillReturn(null);


Isolate.WhenCalled(() => fakeReader.GetInt32(1)).WillReturn(1);