Exception when I start a TaskInstance

[tangomannn]

Hi, and getting a org.hibernate.HibernateException: null index column for collection: org.jbpm.graph.exe.ProcessInstance.instances EXception in the moment I start a Taskinstance (TaskIntance.start() ).
The task instance has already the actor set, and everything seems to be well, but, when I start it, it throws the exception.
Any Idea, or clue?
Thanks in advance
Fernando
The code:

protected void executeFirsTask(){
 JbpmContext ctx = JbpmConfiguration.getInstance().createJbpmContext();
 try{
 List<TaskInstance> tasks = ctx.getTaskMgmtSession().findTaskInstances(getUsername());

 if (tasks!=null && tasks.size() >0){
 TaskInstance ti =tasks.remove(0);
 ti.start(); //HERE Throws the ExceptioN!
 ti.end();
 ctx.save(ti);
 }
 }finally{
 ctx.close();
 }
 }

[kukeltje]

why do you do ti.remove? That is most likely the cause