XF 2.0 Can't delete forum

[Peter Cox]

Getting this error when attemtping to delete a forum - any help appreciated.
---

LogicException: User '279' could not be found in src/XF/Service/User/UserGroupChange.php at line 168

    XF\Service\User\UserGroupChange->applyUserGroupChanges() in src/XF/Service/User/UserGroupChange.php at line 87
    XF\Service\User\UserGroupChange->removeUserGroupChange() in src/XF/Entity/Moderator.php at line 70
    XF\Entity\Moderator->_postDelete() in src/XF/Mvc/Entity/Entity.php at line 1580
    XF\Mvc\Entity\Entity->delete() in src/XF/Entity/ModeratorContent.php at line 67
    XF\Entity\ModeratorContent->_postDelete() in src/XF/Mvc/Entity/Entity.php at line 1580
    XF\Mvc\Entity\Entity->delete() in src/XF/Entity/Node.php at line 306
    XF\Entity\Node->_postDelete() in src/XF/Mvc/Entity/Entity.php at line 1580
    XF\Mvc\Entity\Entity->delete() in src/XF/Admin/Controller/AbstractNode.php at line 117
    XF\Admin\Controller\AbstractNode->nodeDelete() in src/XF/Admin/Controller/AbstractNode.php at line 134
    XF\Admin\Controller\AbstractNode->actionDelete() in src/XF/Mvc/Dispatcher.php at line 321
    XF\Mvc\Dispatcher->dispatchClass() in src/XF/Mvc/Dispatcher.php at line 248
    XF\Mvc\Dispatcher->dispatchFromMatch() in src/XF/Mvc/Dispatcher.php at line 100
    XF\Mvc\Dispatcher->dispatchLoop() in src/XF/Mvc/Dispatcher.php at line 50
    XF\Mvc\Dispatcher->run() in src/XF/App.php at line 2177
    XF\App->run() in src/XF.php at line 390
    XF::runApp() in admin.php at line 13