diff --git a/tests/TestCase.php b/tests/TestCase.php
index 8ad9de8..e6f1e4b 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -33,7 +33,7 @@ abstract class TestCase extends BaseTestCase
         return $user;
     }
 
-    protected function assertFileExistsThenDelete($filePath, $message = null)
+    protected function assertFileExistsThenDelete($filePath, $message = '')
     {
         $this->assertTrue(file_exists($filePath), $message);