diff --git a/src/PHPUnit/Comparator.php b/src/PHPUnit/Comparator.php
index 302bf80..7c65e4e 100644
--- a/src/PHPUnit/Comparator.php
+++ b/src/PHPUnit/Comparator.php
@@ -43,7 +43,7 @@ public function assertEquals($expected, $actual, $delta = 0.0, $canonicalize = f
         );
     }
 
-    private function formatEnum(Enum $enum = null)
+    private function formatEnum(?Enum $enum = null)
     {
         if ($enum === null) {
             return "null";