diff --git a/tests/unit/Functions/Validator/HeadersTest.php b/tests/unit/Functions/Validator/HeadersTest.php
index 4a45f57427..b9cc7c6ea4 100644
--- a/tests/unit/Functions/Validator/HeadersTest.php
+++ b/tests/unit/Functions/Validator/HeadersTest.php
@@ -77,7 +77,7 @@ class HeadersTest extends TestCase
         $this->assertFalse($this->object->isValid($headers));
 
         $headers = [
-            null => 'value',
+            (string) null => 'value',
         ];
         $this->assertFalse($this->object->isValid($headers));