diff --git a/gio/gdbusintrospection.c b/gio/gdbusintrospection.c
index 10e38dd94..3f0c757fe 100644
--- a/gio/gdbusintrospection.c
+++ b/gio/gdbusintrospection.c
@@ -773,7 +773,7 @@ g_dbus_property_info_generate_xml (GDBusPropertyInfo *info,
  * g_dbus_interface_info_generate_xml:
  * @info: A #GDBusNodeInfo
  * @indent: Indentation level.
- * @string_builder: A #GString to to append XML data to.
+ * @string_builder: (out): A #GString to to append XML data to.
  *
  * Appends an XML representation of @info (and its children) to @string_builder.
  *
@@ -822,7 +822,7 @@ g_dbus_interface_info_generate_xml (GDBusInterfaceInfo *info,
  * g_dbus_node_info_generate_xml:
  * @info: A #GDBusNodeInfo.
  * @indent: Indentation level.
- * @string_builder: A #GString to to append XML data to.
+ * @string_builder: (out): A #GString to to append XML data to.
  *
  * Appends an XML representation of @info (and its children) to @string_builder.
  *