Spotted in production:
Invalid or virtual namespace -1 given.
On urls of the format:
/w/api.php?action=flow&format=json&submodule=view-header&uselang=en&page=Special%3AUndelete&vhformat=html
With the following stacktrace:
```
#0 /srv/mediawiki/php-1.30.0-wmf.17/extensions/Flow/includes/WorkflowLoaderFactory.php(76): WikiPage::factory(Title)
#1 /srv/mediawiki/php-1.30.0-wmf.17/extensions/Flow/includes/Api/ApiFlowBase.php(78): Flow\WorkflowLoaderFactory->createWorkflowLoader(Title)
#2 /srv/mediawiki/php-1.30.0-wmf.17/extensions/Flow/includes/Api/ApiFlowBaseGet.php(12): Flow\Api\ApiFlowBase->getLoader()
#3 /srv/mediawiki/php-1.30.0-wmf.17/extensions/Flow/includes/Api/ApiFlow.php(100): Flow\Api\ApiFlowBaseGet->execute()
#4 /srv/mediawiki/php-1.30.0-wmf.17/includes/api/ApiMain.php(1583): Flow\Api\ApiFlow->execute()
#5 /srv/mediawiki/php-1.30.0-wmf.17/includes/api/ApiMain.php(546): ApiMain->executeAction()
#6 /srv/mediawiki/php-1.30.0-wmf.17/includes/api/ApiMain.php(517): ApiMain->executeActionWithErrorHandling()
#7 /srv/mediawiki/php-1.30.0-wmf.17/api.php(94): ApiMain->execute()
#8 /srv/mediawiki/w/api.php(3): include(string)
#9 {main}
```