diff --git zen/template.php zen/template.php
index 29772b3..99b73c9 100644
--- zen/template.php
+++ zen/template.php
@@ -231,7 +231,8 @@ function zen_preprocess_block(&$vars, $hook) {
   $block = $vars['block'];
 
   // Special classes for blocks.
-  $classes = array('block');
+  $classes = isset($vars['classes']) ? $vars['classes'] : array();
+  $classes[] = 'block';
   $classes[] = 'block-' . $block->module;
   $classes[] = 'region-' . $vars['block_zebra'];
   $classes[] = $vars['zebra'];