DoctrineExtensions: Moving a root node up/down throws an error

I’ve got a tree with several root node elements using the nested tree behaviour but when I try to move one of these root level nodes up a level things go borked.

Error: Cannot get siblings from tree root node

#0 /vagrant/src/vendor/gedmo/doctrine-extensions/lib/Gedmo/Tree/Entity/Repository/NestedTreeRepository.php(421): Gedmo\Tree\Entity\Repository\NestedTreeRepository->getNextSiblingsQueryBuilder(Object(Rocket\Entity\Category), false)
#1 /vagrant/src/vendor/gedmo/doctrine-extensions/lib/Gedmo/Tree/Entity/Repository/NestedTreeRepository.php(433): Gedmo\Tree\Entity\Repository\NestedTreeRepository->getNextSiblingsQuery(Object(Rocket\Entity\Category), false)
#2 /vagrant/src/vendor/gedmo/doctrine-extensions/lib/Gedmo/Tree/Entity/Repository/NestedTreeRepository.php(522): Gedmo\Tree\Entity\Repository\NestedTreeRepository->getNextSiblings(Object(Rocket\Entity\Category))
#3 /vagrant/src/application/services/Category.php(65): Gedmo\Tree\Entity\Repository\NestedTreeRepository->moveDown(Object(Rocket\Entity\Category), 1)
#4 /vagrant/src/application/modules/backend/controllers/CategoriesController.php(75): Rocket_Service_Category->moveCategoryDown(Object(Rocket\Entity\Category))
#5 /vagrant/src/vendor/breerly/zf1/library/Zend/Controller/Action.php(516): Backend_CategoriesController->movedownAction()
#6 /vagrant/src/vendor/breerly/zf1/library/Zend/Controller/Dispatcher/Standard.php(295): Zend_Controller_Action->dispatch('movedownAction')
#7 /vagrant/src/vendor/breerly/zf1/library/Zend/Controller/Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#8 /vagrant/src/vendor/breerly/zf1/library/Zend/Application/Bootstrap/Bootstrap.php(97): Zend_Controller_Front->dispatch()
#9 /vagrant/src/vendor/breerly/zf1/library/Zend/Application.php(366): Zend_Application_Bootstrap_Bootstrap->run()
#10 /vagrant/src/public/index.php(43): Zend_Application->run()
#11 {main}