Permalinks won’t give a 404!

My website has a number of Pages at www.domain.com/wp/the-page-name/.

I also have blog posts at www.domain.com/wp/articles/category/the-post-name/, so I have set a custom permalink structure as /articles/%category%/%postname%/

My problem is that I cannot get the site to 404 when it’s supposed to.

www.domain.com/wp/articles gives a list of blog posts.

www.domain.com/wp/articles/random-chars does give a 404.

but

www.domain.com/wp/random-chars gives the list of blog posts. I want a 404 here… what’s going on?