diff --git a/core/lib/Drupal/Core/Render/Placeholder/PlaceholderStrategyManager.php b/core/lib/Drupal/Core/Render/Placeholder/PlaceholderStrategyManager.php
index b24ecc2..0940fd4 100644
--- a/core/lib/Drupal/Core/Render/Placeholder/PlaceholderStrategyManager.php
+++ b/core/lib/Drupal/Core/Render/Placeholder/PlaceholderStrategyManager.php
@@ -40,7 +40,7 @@ public function processPlaceholders(array $placeholders) {
     }
 
     // Assert that there is at least one strategy.
-    assert('!empty($this->placeholderStrategies)', 'At least one placeholder strategy needs to be present.');
+    assert('!empty($this->placeholderStrategies)', 'At least one placeholder strategy must be present; by default the fallback strategy \Drupal\Core\Render\Placeholder\SingleFlushStrategy is always present.');
 
     $new_placeholders = [];
 
@@ -49,7 +49,7 @@ public function processPlaceholders(array $placeholders) {
     // and this uses a variation of the "chain of responsibility" design pattern.
     foreach ($this->placeholderStrategies as $strategy) {
       $processed_placeholders = $strategy->processPlaceholders($placeholders);
-      assert('array_intersect_key($processed_placeholders, $placeholders) === $processed_placeholders', 'Processed placeholders need to be a subset of all placeholders.');
+      assert('array_intersect_key($processed_placeholders, $placeholders) === $processed_placeholders', 'Processed placeholders must be a subset of all placeholders.');
       $placeholders = array_diff_key($placeholders, $processed_placeholders);
       $new_placeholders += $processed_placeholders;
 
diff --git a/core/tests/Drupal/Tests/Core/Render/Placeholder/PlaceholderStrategyManagerTest.php b/core/tests/Drupal/Tests/Core/Render/Placeholder/PlaceholderStrategyManagerTest.php
index 9e2d0f7..387cef5 100644
--- a/core/tests/Drupal/Tests/Core/Render/Placeholder/PlaceholderStrategyManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/Placeholder/PlaceholderStrategyManagerTest.php
@@ -127,7 +127,7 @@ public function providerProcessPlaceholders() {
    * @covers ::processPlaceholders
    *
    * @expectedException \AssertionError
-   * @expectedExceptionMessage At least one placeholder strategy needs to be present.
+   * @expectedExceptionMessage At least one placeholder strategy must be present; by default the fallback strategy \Drupal\Core\Render\Placeholder\SingleFlushStrategy is always present.
    */
   public function testProcessPlaceholdersNoStrategies() {
     // Placeholders but no strategies defined.
@@ -143,7 +143,7 @@ public function testProcessPlaceholdersNoStrategies() {
    * @covers ::processPlaceholders
    *
    * @expectedException \AssertionError
-   * @expectedExceptionMessage Processed placeholders need to be a subset of all placeholders.
+   * @expectedExceptionMessage Processed placeholders must be a subset of all placeholders.
    */
   public function testProcessPlaceholdersWithRoguePlaceholderStrategy() {
     // Placeholders but no strategies defined.