I'm trying to add a specific page type via the Sitemap and am getting this error when trying to publish:

You haven't chosen a valid URL Slug

The URL slug looks fine on first look. For example the page name is 'Test Project' and the URL is 'test-project'.

The interesting part is that it only happens when I add this page type via the Sitemap. If I add it via the new page icon at the top right of the Concrete5 Toolbar, it works fine. Although when you add it that way, it runs through the composer. When I add a page via the Sitemap, it's just the standard page addition. I've attached an image of the error message for reference.

Any ideas?