Soft 404 errors on posts page

Hi, I’ve got a WordPress site setup with a static front page and my posts page points to /blog/.

I recently deleted a post within the blog, however, when I hit the old URL (http://www.example.com/blog?p=388), it displays the “File or Page Not Found” message that’s on my index.php template file (the else to the “if(have_posts())”) but it’s returning a 200 status code, rather than 404.

When I hit the same post ID off the root of the site (http://www.example.com/?p=388), I get taken to the correct 404 page.

I found this which in theory should fix: http://wordpress.stackexchange.com/a/46123

But I’m just wondering if there’s something I’m missing which would resolve this issue, rather than having to make a fix into the theme.