Template Route Manager, 'Segments Required'- if segments aren't present, then show 404

I have set up a new template group which is going to provide templates for an ajax call made with the jQuery plugin ‘Magnific Popup’.

The URL structure will look like:

/ajax/{name}/{id}

I have set the ‘segments required’ option to ‘yes’ in the Template Route Manager. It seems to be working, but when any of the segments are missing, it just shows a blank page, rather than a 404. Why? Can I set this somewhere to show a 404?

Bonus points: limit this route to only ajax requests, and show a 404 for any normal get requests. (can’t see any easy way of doing this either- is there a plugin perhaps?)

Thanks.