diff --git a/MysqliDb.php b/MysqliDb.php
index cea80ae..6795f6b 100644
--- a/MysqliDb.php
+++ b/MysqliDb.php
@@ -847,7 +847,7 @@ public function insert($tableName, $insertData)
      * @return bool|array Boolean indicating the insertion failed (false), else return id-array ([int])
      * @throws Exception
      */
-    public function insertMulti($tableName, array $multiInsertData, array $dataKeys = null)
+    public function insertMulti($tableName, array $multiInsertData,?array $dataKeys = null)
     {
         // only auto-commit our inserts, if no transaction is currently running
         $autoCommit = (isset($this->_transaction_in_progress) ? !$this->_transaction_in_progress : true);