Requests are not versionable, but they are checked out by the workflow manager

From the logs:

javax.jcr.UnsupportedRepositoryOperationException: Unable to perform a versioning operation on a non versionable node: node /content/documents/articles/myarticle2/hippo:request
javax.jcr.UnsupportedRepositoryOperationException: Unable to perform a versioning operation on a non versionable node: node /content/documents/articles/myarticle2/hippo:request
at org.apache.jackrabbit.core.NodeImpl.checkVersionable(NodeImpl.java:3556)
at org.apache.jackrabbit.core.NodeImpl.checkout(NodeImpl.java:3314)
at org.hippoecm.repository.decorating.NodeDecorator.checkout(NodeDecorator.java:389)
at org.hippoecm.repository.impl.NodeDecorator.checkout(NodeDecorator.java:172)
at org.hippoecm.repository.impl.DocumentManagerImpl.putObject(DocumentManagerImpl.java:141)
at org.hippoecm.repository.impl.WorkflowManagerImpl$WorkflowInvocationHandler.invoke(WorkflowManagerImpl.java:479)
at $Proxy5.hints(Unknown Source)
at org.hippoecm.repository.impl.WorkflowDescriptorImpl.<init>(WorkflowDescriptorImpl.java:90)
at org.hippoecm.repository.impl.WorkflowDescriptorImpl.<init>(WorkflowDescriptorImpl.java:56)
at org.hippoecm.repository.impl.WorkflowManagerImpl.getWorkflowDescriptor(WorkflowManagerImpl.java:266)
at org.hippoecm.repository.decorating.checked.WorkflowManagerDecorator.getWorkflowDescriptor(WorkflowManagerDecorator.java:52)
at org.hippoecm.addon.workflow.AbstractWorkflowPlugin.buildMenu(AbstractWorkflowPlugin.java:87)
at org.hippoecm.addon.workflow.WorkflowsPlugin.onModelChanged(WorkflowsPlugin.java:112)
at org.hippoecm.addon.workflow.WorkflowsPlugin$2.onEvent(WorkflowsPlugin.java:101)
at org.hippoecm.frontend.model.event.ObservableRegistry$ObservationContext.notifyObservers(ObservableRegistry.java:76)
at org.hippoecm.frontend.model.event.JcrEventListener.onEvent(JcrEventListener.java:62)