NPE in Serialization if annotation @XStreamAsAttribute is used with a generic

We are using quite heavily xstream, with xstream 1.4.x we are not able to serialize our classes anymore.
Problem is the @XStreamAsAttribute on a generic. It works with 1.3.1
Attached are testcases for it. Currently this is a blocker for us for updating xstream to any newer version. Without the annotation and its processing it works.
for example
....
public class Test1<VALTYPE extends Serializable> implements Serializable
{

private static final long serialVersionUID = 1383196364083356677L;
@XStreamAsAttribute
private VALTYPE value;
......
results in an NPE in
com.thoughtworks.xstream.converters.reflection.SerializationMethodInvoker.getMethod(SerializationMethodInvoker.java:161)