when I run the test case org.jbpm.jcr.JcrDbTest , hibernate session can't be committed.

see below key lines:

....deploy process.
....
ProcessInstance processInstance = jbpmContext.newProcessInstanceForUpdate("document approval");
.....
Node documentNode = processInstanceNode.addNode("document");
.....
contextInstance.setVariable("document", documentNode);
//comment the line above, there's no exception.
....

According to the chapter 12 titled 'Document management' of user guide, it said:

"Next, JCR Node's can be stored in the process variables"

that seems doesn't work!

I also viewed the class file 'JcrNodeInstance' and found it extends class 'VariableInstance',
the node-type variable ought support persistent.
something else wrong?

Details please see the attachment 'jsr-cannot-persistent.txt'