diff --git a/grade/edit/tree/calculation.php b/grade/edit/tree/calculation.php
index 21e2d7dd11f..bd275513d8a 100644
--- a/grade/edit/tree/calculation.php
+++ b/grade/edit/tree/calculation.php
@@ -30,7 +30,7 @@ require_once 'calculation_form.php';
 $courseid  = required_param('courseid', PARAM_INT);
 $id        = required_param('id', PARAM_INT);
 $section   = optional_param('section', 'calculation', PARAM_ALPHA);
-$idnumbers = optional_param('idnumbers', null, PARAM_RAW);
+$idnumbers = optional_param_array('idnumbers', null, PARAM_RAW);
 
 $url = new moodle_url('/grade/edit/tree/calculation.php', array('id'=>$id, 'courseid'=>$courseid));
 if ($section !== 'calculation') {