diff --git a/lib/dml/tests/fixtures/read_slave_moodle_database.php b/lib/dml/tests/fixtures/read_slave_moodle_database.php
index 6029e91a620..982dbad135d 100644
--- a/lib/dml/tests/fixtures/read_slave_moodle_database.php
+++ b/lib/dml/tests/fixtures/read_slave_moodle_database.php
@@ -200,7 +200,7 @@ class read_slave_moodle_database extends test_moodle_database {
      * @param string $dbh
      * @return void
      */
-    protected function set_db_handle($dbh) {
+    protected function set_db_handle($dbh): void {
         $this->handle = $dbh;
     }