Can't retrieve Activiti 6 Process in BPMN format (Activiti 5 compatibility error)

I am using activiti 6 (alfresco) and can’t retrieve my BPMN in activiti 6 format, I end up getting the error

org.activiti.engine.ActivitiException: Found Activiti 5 process definition, but no compatibility handler on the classpath

Here is the code to insert (+ a test to check data validity):

RepositoryService repositoryService = processEngine.getRepositoryService();
BpmnXMLConverter converter = new BpmnXMLConverter();
BpmnModel bpmnModel = converter.convertToBpmnModel(xtr);

Collection<FlowElement> lFlows = bpmnModel.getProcesses().get(0).getFlowElements();//==> works fine

Deployment deployment = repositoryService.createDeployment().addBpmnModel(processName, bpmnModel).deploy();

Here is the code to retrieve the model:

RepositoryService repositoryService = processEngine.getRepositoryService();
        List<ProcessDefinition> lpd = repositoryService.createProcessDefinitionQuery().active().list();
for (ProcessDefinition processDefition : lpd) {
    org.activiti.bpmn.model.Process process = ProcessDefinitionUtil.getProcess(processDefition.getId());// ==> error
}

I tried also:

BpmnModel bpmnModel = ProcessDefinitionUtil.getBpmnModel(processDefition.getId());

But I got the same error.

Thanks for your help.