diff -u b/themekey_admin.inc b/themekey_admin.inc
--- b/themekey_admin.inc
+++ b/themekey_admin.inc
@@ -718,7 +718,7 @@
         // Add special classes to [old_items] additional columns.
         if (isset($options['additional_fields'])) {
           foreach ($options['additional_fields'] as $field_name) {
-            $form['old_items'][$key]['parent']['#attributes']['class'] = array("themekey-property-$field_name");
+            $form['old_items'][$key][$field_name]['#attributes']['class'] = array(drupal_clean_css_identifier("themekey-property-$field_name"));
           }
         }
 
@@ -852,7 +852,7 @@
       // Add special classes to [new_item] additional columns.
       if (isset($options['additional_fields'])) {
         foreach ($options['additional_fields'] as $field_name) {
-          $form['new_item'][$field_name]['#attributes']['class'] = array("themekey-property-$field_name");
+          $form['new_item'][$field_name]['#attributes']['class'] = array(drupal_clean_css_identifier("themekey-property-$field_name"));
         }
       }