diff --git a/tests/system/CommonFunctionsTest.php b/tests/system/CommonFunctionsTest.php
index 98ee02f30bbc..3dd8cfe3cfe8 100644
--- a/tests/system/CommonFunctionsTest.php
+++ b/tests/system/CommonFunctionsTest.php
@@ -391,7 +391,7 @@ public function testReallyWritable()
     public function testSlashItem()
     {
         $this->assertSame('/', slash_item('cookiePath')); // slash already there
-        $this->assertSame('', slash_item('cookieDomain')); // empty, so untouched
+        $this->assertNull(null, slash_item('cookieDomain')); // empty, so untouched
         $this->assertSame('en/', slash_item('defaultLocale')); // slash appended
     }