diff --git a/core/themes/olivero/templates/navigation/book-tree.html.twig b/core/themes/olivero/templates/navigation/book-tree.html.twig
index 4a4ffc7de558b43a2be9b274ec911fb3bc5ad680..4daf61884cf46aca160e91257bcdd11e2b626913 100644
--- a/core/themes/olivero/templates/navigation/book-tree.html.twig
+++ b/core/themes/olivero/templates/navigation/book-tree.html.twig
@@ -32,7 +32,7 @@
     {% if menu_level == 0 %}
       <ul{{ attributes.addClass('book-navigation__menu menu', 'menu--level-' ~ (menu_level + 1)) }}>
     {% else %}
-      <ul class='menu menu--level-'  ~ (menu_level + 1)>
+      <ul class='menu menu--level-{{ menu_level + 1 }}'>
     {% endif %}
     {% for item in items %}
       {%