diff --git a/mod/assign/renderer.php b/mod/assign/renderer.php
index 6803fd1565a..3bef77e299a 100644
--- a/mod/assign/renderer.php
+++ b/mod/assign/renderer.php
@@ -55,7 +55,7 @@ class mod_assign_renderer extends plugin_renderer_base {
      * @return string
      */
     public function render_assign_files(assign_files $tree) {
-        $this->htmlid = 'assign_files_tree_'.uniqid();
+        $this->htmlid = html_writer::random_id('assign_files_tree');
         $this->page->requires->js_init_call('M.mod_assign.init_tree', array(true, $this->htmlid));
         $html = '<div id="'.$this->htmlid.'">';
         $html .= $this->htmllize_tree($tree, $tree->dir);