Why does force https for login/admin pages setting not save?

Hi,

I've got my domain mapping settings configured as such:

Administration mapping: original domain
Login mapping: original domain
Cross-domain autologin: yes

Force http/https (Only for original domain):
- Force http for frontend pages
- Force https in login and admin pages <<<<

When I save the changes on that settings page, the "Force https in login and admin pages" option refuses to be saved as "Yes" and instead always reverts to "No".

Is there anything special that I need to configure/change?