diff -u b/core/modules/simpletest/tests/src/Unit/TestBaseTest.php b/core/modules/simpletest/tests/src/Unit/TestBaseTest.php
--- b/core/modules/simpletest/tests/src/Unit/TestBaseTest.php
+++ b/core/modules/simpletest/tests/src/Unit/TestBaseTest.php
@@ -317,8 +317,6 @@
       // Null.
       [FALSE, FALSE, 'NULL', NULL],
       [TRUE, TRUE, NULL, NULL],
-      // Null against unset value.
-      [TRUE, TRUE, $x, NULL],
     ];
   }