Safari asking for certificate confirmation whereas other browsers don't.

I am using Safari 5.1.7 on Windows 7. When trying to access on of our https sites i get the "Confirm the certificate by clicking OK" error message. This does not happen on any other browser(Chrome/Firefox/IE) as we have a setting on our servers to say if client browser has certificate then send it otherwise don't bother. All the browsers are honoring this except it seems Safari. Any ideas on what could be causing this issue.