diff --git a/subjects/grades/index.php b/subjects/grades/index.php
index a8a9414..395848f 100644
--- a/subjects/grades/index.php
+++ b/subjects/grades/index.php
@@ -133,7 +133,7 @@
     $average = $weight_otherum / $weight_sum;
     // Insert average into subject
     if ($stmt = $con->prepare('UPDATE subjects SET average = ? WHERE id = ?')) {
-        $stmt->bind_param('si', $average, $subject_id);
+        $stmt->bind_param('ss', $average, $subject_id);
         $stmt->execute();
         $stmt->close();
     } else {
@@ -143,7 +143,7 @@
 // If no grades but average is given -> delete average
 if ($num_of_k + $num_of_m + $num_of_t + $num_of_s == 0) {
     if ($stmt = $con->prepare('UPDATE subjects SET average = 0 WHERE id = ?')) {
-        $stmt->bind_param('i', $subject_id);
+        $stmt->bind_param('s', $subject_id);
         $stmt->execute();
         $stmt->close();
     } else {