diff --git a/mod/assign/classes/output/renderer.php b/mod/assign/classes/output/renderer.php
index 2d7afe12e8d..9f2e8aeabe6 100644
--- a/mod/assign/classes/output/renderer.php
+++ b/mod/assign/classes/output/renderer.php
@@ -1443,7 +1443,8 @@ class renderer extends \plugin_renderer_base {
         ];
         $button->set_callback_options('assign_portfolio_caller', $portfolioparams, 'mod_assign');
         $button->set_format_by_file($file);
-        return $button->to_html(PORTFOLIO_ADD_ICON_LINK);
+
+        return (string) $button->to_html(PORTFOLIO_ADD_ICON_LINK);
     }
 
     /**