Force Secure URLs (SSL) not working despite SSL cert

I have an SSL certificate for my site, but cannot set 'Force Secure URLs (SSL)' (in Resource URLs) to Yes. I get the message 'You need to set up a secure server before you can use the Force Secure URLs option'. SSL is installed and working and it doesn't seem possible to access the system except through https. This is a Drupal install with CONF('https') left to default.