diff -u b/tests/src/Functional/ReadOnlyConfigTest.php b/tests/src/Functional/ReadOnlyConfigTest.php
--- b/tests/src/Functional/ReadOnlyConfigTest.php
+++ b/tests/src/Functional/ReadOnlyConfigTest.php
@@ -32,7 +32,7 @@
    *
    * @var string
    */
-  private $message = 'This form will not be saved because the configuration active store is read-only.';
+  protected $message = 'This form will not be saved because the configuration active store is read-only.';
 
   /**
    * {@inheritdoc}