CERT_SUBJECT populating in Chrome but not IE 9

I've got an ASPX site that pulls the ServerVariables. In particular we've been using CERT_SUBJECT to get the EDIPI. This has worked for both Chrome and IE in the past but now (not sure when it broke) it doesn't work under IE. In IE the CERT_SUBJECT variable is null. In Chrome it is populated like expected. I've looked around in IIS but cannot seem to find the answer.
Edit: I just tried CERT_SERVER_SUBJECT and it worked both on Chrome and IE9. For some reason CERT_SUBJECT doesn't.

Can anyone please point me in the right direction?

Thanks