Email sends with blank merge fields.

So I'm working on an application that includes some basic e-mail functionality. I've created an e-mail template and a custom object called EmailMessage__c with fields Subject__c and Body__c which are referenced in the template.

When I test it logged in as a system administrator, it works fine. But when I log in through the portal frontend as an end user (a profile based on "Overage High Volume Customer Portal"), the e-mail sends with a blank subject and body. Querying the database confirms that the EmailMessage__c object was saved and committed to the database with the correct values, but they weren't filled in.

I can only assume that this is a permissions issue somewhere, but I can't find anything wrong. Both System Administrator and our profile based on Overage High Volume Customer Portal have full permissions on the EmailMessage__c object, and I also ensured that it was listed as available for the customer portal, but it still sends blank messages.

Any idea what's going on?