diff --git a/core/modules/book/tests/src/Functional/BookTest.php b/core/modules/book/tests/src/Functional/BookTest.php
index 53afc1a..7220146 100644
--- a/core/modules/book/tests/src/Functional/BookTest.php
+++ b/core/modules/book/tests/src/Functional/BookTest.php
@@ -255,7 +255,7 @@ function checkBookNode(EntityInterface $node, $nodes, $previous = FALSE, $up = F
 
     // Check outline structure.
     if ($nodes !== NULL) {
-      $this->assertPattern($this->generateOutlinePattern($nodes));
+      $this->assertPattern($this->generateOutlinePattern($nodes), format_string('Node @number outline confirmed.', array('@number' => $number)));
     }
     else {
       $this->pass(format_string('Node %number does not have outline.', array('%number' => $number)));
diff --git a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
index ad42806..b18026a 100644
--- a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
+++ b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
@@ -208,11 +208,16 @@ protected function assertNoRaw($raw) {
    *
    * @param string $pattern
    *   Perl regex to look for including the regex delimiters.
+   * @param string $message
+   *   (optional) A message to display with the assertion. Do not translate
+   *   messages: use \Drupal\Component\Utility\SafeMarkup::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    *
    * @deprecated Scheduled for removal in Drupal 9.0.0.
    *   Use $this->assertSession()->responseMatches() instead.
    */
-  protected function assertPattern($pattern) {
+  protected function assertPattern($pattern, $message = '') {
     $this->assertSession()->responseMatches($pattern);
   }