diff --git a/modules/book.module b/modules/book.module
index 9a0c736285bf470529920c6778ab6a469330f1bf..2ed3f5c9ed036a69b94ef458d00cba446cb5ea4d 100644
--- a/modules/book.module
+++ b/modules/book.module
@@ -578,7 +578,7 @@ function book_tree_recurse($nid, $depth, $children, $unfold = array()) {
           if ($tree = book_tree_recurse($node->nid, $depth - 1, $children, $unfold)) {
             $output .= '<li class="expanded">';
             $output .= l($node->title, 'node/'. $node->nid);
-            $output .= '<ul>'. $tree .'</ul>';
+            $output .= '<ul class="menu">'. $tree .'</ul>';
             $output .= '</li>';
           }
           else {
diff --git a/modules/book/book.module b/modules/book/book.module
index 9a0c736285bf470529920c6778ab6a469330f1bf..2ed3f5c9ed036a69b94ef458d00cba446cb5ea4d 100644
--- a/modules/book/book.module
+++ b/modules/book/book.module
@@ -578,7 +578,7 @@ function book_tree_recurse($nid, $depth, $children, $unfold = array()) {
           if ($tree = book_tree_recurse($node->nid, $depth - 1, $children, $unfold)) {
             $output .= '<li class="expanded">';
             $output .= l($node->title, 'node/'. $node->nid);
-            $output .= '<ul>'. $tree .'</ul>';
+            $output .= '<ul class="menu">'. $tree .'</ul>';
             $output .= '</li>';
           }
           else {