diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index e27af49..e17b52c 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -516,7 +516,8 @@ function template_preprocess_block(&$variables) {
   $variables['content'] = $variables['elements']['content'];
 
   $variables['attributes']['class'][] = 'block';
-  $variables['attributes']['class'][] = drupal_html_class('block-' . $variables['configuration']['module']);
+  $variables['attributes']['class'][] = drupal_html_class($variables['configuration']['module']);
+  $variables['attributes']['class'][] = drupal_html_class($variables['plugin_id']);
 
   // The block template provides a wrapping element for the content. Render the
   // #attributes of the content on this wrapping element rather than passing