diff --git a/docs/source/reference/_examples/simple_dashboard.py b/docs/source/reference/_examples/simple_dashboard.py
index 540082f58..81e696489 100644
--- a/docs/source/reference/_examples/simple_dashboard.py
+++ b/docs/source/reference/_examples/simple_dashboard.py
@@ -83,7 +83,7 @@ def update_value(value):
         set_value_callback(value)
 
     return idom.html.fieldset(
-        {"class": "number-input-container"},
+        {"class_name": "number-input-container"},
         idom.html.legend({"style": {"font-size": "medium"}}, label),
         Input(update_value, "number", value, attributes=attrs, cast=float),
         Input(update_value, "range", value, attributes=attrs, cast=float),