diff --git a/mod/quiz/report/statistics/db/upgrade.php b/mod/quiz/report/statistics/db/upgrade.php
index f4e4764b12d0..07375c41e90c 100644
--- a/mod/quiz/report/statistics/db/upgrade.php
+++ b/mod/quiz/report/statistics/db/upgrade.php
@@ -161,7 +161,7 @@ function xmldb_quiz_statistics_upgrade($oldversion) {
     }
 
     if ($oldversion < 2008112101) {
-        // Removed stupid UNSIGNED from all NUMBER columns in the quiz_statistics table.
+        // Removed UNSIGNED from all NUMBER columns in the quiz_statistics table.
         $table = new xmldb_table('quiz_statistics');
 
         // Change of sign for field firstattemptsavg
@@ -172,19 +172,19 @@ function xmldb_quiz_statistics_upgrade($oldversion) {
         $field = new xmldb_field('allattemptsavg', XMLDB_TYPE_NUMBER, '15, 5', null, null, null, null, 'firstattemptsavg');
         $dbman->change_field_unsigned($table, $field);
 
-                // Change of sign for field median
+        // Change of sign for field median
         $field = new xmldb_field('median', XMLDB_TYPE_NUMBER, '15, 5', null, null, null, null, 'allattemptsavg');
         $dbman->change_field_unsigned($table, $field);
 
-                // Change of sign for field standarddeviation
+        // Change of sign for field standarddeviation
         $field = new xmldb_field('standarddeviation', XMLDB_TYPE_NUMBER, '15, 5', null, null, null, null, 'median');
         $dbman->change_field_unsigned($table, $field);
 
-                // Change of sign for field errorratio
+        // Change of sign for field errorratio
         $field = new xmldb_field('errorratio', XMLDB_TYPE_NUMBER, '15, 10', null, null, null, null, 'cic');
         $dbman->change_field_unsigned($table, $field);
 
-                // Change of sign for field standarderror
+        // Change of sign for field standarderror
         $field = new xmldb_field('standarderror', XMLDB_TYPE_NUMBER, '15, 10', null, null, null, null, 'errorratio');
         $dbman->change_field_unsigned($table, $field);
 
@@ -193,7 +193,7 @@ function xmldb_quiz_statistics_upgrade($oldversion) {
     }
 
     if ($oldversion < 2008112102) {
-        // Removed stupid UNSIGNED from all NUMBER columns in the quiz_question_statistics table.
+        // Removed UNSIGNED from all NUMBER columns in the quiz_question_statistics table.
         $table = new xmldb_table('quiz_question_statistics');
 
         // Change of sign for field effectiveweight
@@ -204,11 +204,11 @@ function xmldb_quiz_statistics_upgrade($oldversion) {
         $field = new xmldb_field('sd', XMLDB_TYPE_NUMBER, '15, 10', null, null, null, null, 'discriminativeefficiency');
         $dbman->change_field_unsigned($table, $field);
 
-                // Change of sign for field facility
+        // Change of sign for field facility
         $field = new xmldb_field('facility', XMLDB_TYPE_NUMBER, '15, 10', null, null, null, null, 'sd');
         $dbman->change_field_unsigned($table, $field);
 
-                // Change of sign for field maxgrade
+        // Change of sign for field maxgrade
         $field = new xmldb_field('maxgrade', XMLDB_TYPE_NUMBER, '12, 7', null, null, null, null, 'subquestions');
         $dbman->change_field_unsigned($table, $field);
 
@@ -221,7 +221,7 @@ function xmldb_quiz_statistics_upgrade($oldversion) {
         $table = new xmldb_table('quiz_question_response_stats');
 
         // Change of sign for field credit
-        $field = new xmldb_field('credit', XMLDB_TYPE_NUMBER, '15, 5', null, null, null, null, 'rcount');
+        $field = new xmldb_field('credit', XMLDB_TYPE_NUMBER, '15, 5', null, XMLDB_NOTNULL, null, null, 'rcount');
         $dbman->change_field_unsigned($table, $field);
 
         // statistics savepoint reached