diff --git a/src/SphinxQL.php b/src/SphinxQL.php
index ddcd3e6..cfc7bc4 100644
--- a/src/SphinxQL.php
+++ b/src/SphinxQL.php
@@ -304,7 +304,7 @@ public function executeBatch()
      *
      * @return SphinxQL A new SphinxQL object with the current object referenced
      */
-    public function enqueue(SphinxQL $next = null)
+    public function enqueue(?SphinxQL $next = null)
     {
         if ($next === null) {
             $next = new static($this->getConnection());