diff --git a/includes/theme.inc b/includes/theme.inc
index 328c707d349..6ddf2574713 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -2250,7 +2250,7 @@ function template_preprocess(&$variables, $hook) {
   $variables['directory'] = path_to_theme();
 
   // Initialize html class attribute for the current hook.
-  $variables['classes_array'] = array($hook);
+  $variables['classes_array'] = array(drupal_html_class($hook));
 
   // Initialize attributes for the top-level template entity and its title and
   // content.