diff --git a/tests/Internal/RejectedPromiseTest.php b/tests/Internal/RejectedPromiseTest.php
index 38eb3b40..028ca4dc 100644
--- a/tests/Internal/RejectedPromiseTest.php
+++ b/tests/Internal/RejectedPromiseTest.php
@@ -34,7 +34,7 @@ public function getPromiseTestAdapter(callable $canceller = null)
                     $promise = new RejectedPromise($reason);
                 }
             },
-            'settle' => function ($reason = null) use (&$promise) {
+            'settle' => function ($reason = "") use (&$promise) {
                 if (!$promise) {
                     if (!$reason instanceof Exception) {
                         $reason = new Exception($reason);