diff --git a/src/wp-includes/widgets.php b/src/wp-includes/widgets.php
index db06222a51..5dececaaeb 100644
--- a/src/wp-includes/widgets.php
+++ b/src/wp-includes/widgets.php
@@ -704,7 +704,7 @@ function dynamic_sidebar( $index = 1 ) {
 
 	$sidebar = $wp_registered_sidebars[ $index ];
 
-	$sidebar['before_sidebar'] = sprintf( $sidebar['before_sidebar'], $sidebar['id'], $sidebar['class'] );
+	$sidebar['before_sidebar'] = sprintf( $sidebar['before_sidebar'], 'sidebar-' . $sidebar['id'], $sidebar['class'] );
 
 	/**
 	 * Fires before widgets are rendered in a dynamic sidebar.