diff --git a/agaric/templates/block/block.html.twig b/agaric/templates/block/block.html.twig
index 7a5788a..7168abe 100644
--- a/agaric/templates/block/block.html.twig
+++ b/agaric/templates/block/block.html.twig
@@ -33,7 +33,7 @@
         {% if label %}
         <div class="titlebar titlebar-bigborder content">
           <div class="titlebar_title">
-            <h3{{ title_attributes }}.addClass(['subtitle', 'is-mono', 'is-4']>{{ label }}</h3>
+            <h3{{ title_attributes.addClass(['subtitle', 'is-mono', 'is-4']) }}>{{ label }}</h3>
           </div>
         </div>
         {% endif %}