[stunnel-users] IE tries to verify client certificate?

I've got stunnel providing SSL wrapper services for an otherwise SSL-stupid vendor-webserver-thingy that listens on port 80. I'm using a wildcard certificate purchased from $VENDOR, and it seems to work fine in Safari, Firefox, w3m, and basically everything SSL aware known to Man.

Except, that is, Internet Explorer, where the user gets a popup window asking them to choose a certificate to verify their connection.

This is really throwing me for a loop; I've tried setting 'verify = 0' and 'options = ALL' in the service definition under stunnel, no dice. Any ideas why IE thinks it needs to verify, but nothing else does?