diff --git a/mod/quiz/edit.php b/mod/quiz/edit.php
index 61d9b4f1a95..12abf0da1f3 100644
--- a/mod/quiz/edit.php
+++ b/mod/quiz/edit.php
@@ -159,8 +159,8 @@ if ((optional_param('addrandom', false, PARAM_BOOL)) && confirm_sesskey()) {
 if (optional_param('savechanges', false, PARAM_BOOL) && confirm_sesskey()) {
 
     // If rescaling is required save the new maximum.
-    $maxgrade = unformat_float(optional_param('maxgrade', -1, PARAM_RAW));
-    if ($maxgrade >= 0) {
+    $maxgrade = unformat_float(optional_param('maxgrade', '', PARAM_RAW_TRIMMED), true);
+    if (is_float($maxgrade) && $maxgrade >= 0) {
         quiz_set_grade($maxgrade, $quiz);
         quiz_update_all_final_grades($quiz);
         quiz_update_grades($quiz, 0, true);