diff --git a/lib/simpletestlib.php b/lib/simpletestlib.php
index 7af88779d21..4646ee1541f 100644
--- a/lib/simpletestlib.php
+++ b/lib/simpletestlib.php
@@ -186,7 +186,7 @@ class MoodleUnitTestCase extends UnitTestCase {
                 continue;
             }
 
-            if ($max_id = $DB->get_field_sql("SELECT MAX(id) FROM {$CFG->prefix}{$table}")) {
+            if ($max_id = $DB->get_field_sql("SELECT MAX(id) FROM {$CFG->unittest_prefix}{$table}")) {
                 $this->tables[$table] = $max_id;
             } else {
                 $this->tables[$table] = 0;