Problem with adding additional classes to block using Skinr

I just started using Danland and I really like it so far (especially the handbook page).

My problem is that when I go to a block configuration page and try to apply an additional CSS class, that class isn't added to the block. I enter a new class, click save block at the bottom, and receive no error messages. When I go back to that same block's configuration, the additional class I added is displayed in the form field (so it is in fact saving the class), but when I go to the page source the additional class doesn't show up in the HTML.

I have the Skinr module enabled, and I've cleared the theme registry and refreshed but the class still won't show up.

Is there a configuration or setting that I need to change to make this work? The block I want to add a class to is in the preface region but I've tried it in various regions and none have worked.

Thank you in advance for your help.