/?id= not linking to 404 error?

I have a problem that any ID url is linking to my home page and creating duplicates all over the place.

http://www.mysite.com/?id=213213
http://www.mysite.com/?id=213
http://www.mysite.com/?id=2134

At first i tried to find the specific post id etc, but then realised its anything!

http://www.mysite.com/?id=anythinghere
http://www.mysite.com/?id=notjustnumbers
http://www.mysite.com/?id=thisisreallyannoying

None of these go to the correct 404 page, they just link to the home page.

Current permalinks are set to post name and i’ve tried all variations of that.

Any ideas?