diff --git a/core/lib/Drupal/Component/Utility/PlaceholderTrait.php b/core/lib/Drupal/Component/Utility/PlaceholderTrait.php
index ce49f8a..a95cd40 100644
--- a/core/lib/Drupal/Component/Utility/PlaceholderTrait.php
+++ b/core/lib/Drupal/Component/Utility/PlaceholderTrait.php
@@ -107,7 +107,7 @@ protected static function placeholderFormat($string, array $args, &$safe = TRUE)
           break;
 
         default:
-          throw new \InvalidArgumentException('Invalid placeholder');
+          trigger_error('Invalid placeholder', E_USER_ERROR);
           break;
       }
     }
diff --git a/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php b/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
index 9468fe7..6e3da90 100644
--- a/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
@@ -22,6 +22,20 @@
 class SafeMarkupTest extends UnitTestCase {
 
   /**
+   * The error message of the last error in the error handler.
+   *
+   * @var string
+   */
+  protected $lastErrorMessage;
+
+  /**
+   * The error number of the last error in the error handler.
+   *
+   * @var int
+   */
+  protected $lastErrorNumber;
+
+  /**
    * {@inheritdoc}
    */
   protected function tearDown() {
@@ -192,15 +206,38 @@ function providerCheckPlain() {
   }
 
   /**
+   * Custom error handler that saves the last error.
+   *
+   * We need this custom error handler because we cannot rely on the error to
+   * exception conversion as __toString is never allowed to leak any kind of
+   * exception.
+   *
+   * @param int $error_number
+   *   The error number.
+   * @param string $error_message
+   *   The error message.
+   */
+  public function errorHandler($error_number, $error_message) {
+    $this->lastErrorNumber = $error_number;
+    $this->lastErrorMessage = $error_message;
+  }
+
+  /**
    * String formatting with SafeMarkup::format() and an unsupported placeholder.
    *
    * When you call SafeMarkup::format() with an unsupported placeholder, an
    * InvalidArgumentException should be thrown.
-   *
-   * @expectedException \InvalidArgumentException
    */
   public function testUnexpectedFormat() {
-    SafeMarkup::format('Broken placeholder: ~placeholder', ['~placeholder' => 'broken'])->__toString();
+
+    // We set a custom error handler because of https://github.com/sebastianbergmann/phpunit/issues/487
+    set_error_handler([$this, 'errorHandler']);
+    // We want this to trigger an error.
+    $error = SafeMarkup::format('Broken placeholder: ~placeholder', ['~placeholder' => 'broken'])->__toString();
+    restore_error_handler();
+
+    $this->assertEquals(E_USER_ERROR, $this->lastErrorNumber);
+    $this->assertEquals('Invalid placeholder', $this->lastErrorMessage);
   }
 
   /**