NPE when marshaling task data

When executing the evaluation example (using jbpm-installer) but completing the task using the human task view (which means no result data is returned), I get the following NPE:

Caused by: java.lang.NullPointerException
at org.drools.marshalling.impl.ClassObjectMarshallingStrategyAcceptor.accept(ClassObjectMarshallingStrategyAcceptor.java:47)
at org.drools.marshalling.impl.SerializablePlaceholderResolverStrategy.accept(SerializablePlaceholderResolverStrategy.java:59)
at org.drools.marshalling.impl.ObjectMarshallingStrategyStoreImpl.getStrategyObject(ObjectMarshallingStrategyStoreImpl.java:72)
at org.jbpm.marshalling.impl.ProtobufProcessMarshaller.marshallVariablesMap(ProtobufProcessMarshaller.java:234)
at org.jbpm.task.utils.ContentMarshallerHelper.marshal(ContentMarshallerHelper.java:60)
at org.jbpm.process.workitem.wsht.AbstractHTWorkItemHandler.createTaskContentBasedOnWorkItemParams(AbstractHTWorkItemHandler.java:160)
at org.jbpm.process.workitem.wsht.GenericHTWorkItemHandler.executeWorkItem(GenericHTWorkItemHandler.java:168)
at org.drools.process.instance.impl.DefaultWorkItemManager.internalExecuteWorkItem(DefaultWorkItemManager.java:70)
at org.jbpm.workflow.instance.node.WorkItemNodeInstance.internalTrigger(WorkItemNodeInstance.java:107)
at org.jbpm.workflow.instance.impl.NodeInstanceImpl.trigger(NodeInstanceImpl.java:126)
... 18 more