diff --git a/menu_block.module b/menu_block.module
index 83cf0c9..8d9b988 100644
--- a/menu_block.module
+++ b/menu_block.module
@@ -140,7 +140,7 @@ function menu_block_block_view($delta = '') {
 function template_preprocess_menu_block_wrapper(&$variables) {
   $variables['classes_array'][] = 'menu-block-' . $variables['delta'];
   $variables['classes_array'][] = 'menu-name-' . $variables['config']['menu_name'];
-  $variables['classes_array'][] = 'parent-mlid-' . $variables['config']['parent_mlid'];
+  $variables['classes_array'][] = 'parent-mlid-' . str_replace(':', '-', $variables['config']['parent_mlid']);
   $variables['classes_array'][] = 'menu-level-' . $variables['config']['level'];
 }