diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Tests/AggregatorRenderingTest.php b/core/modules/aggregator/lib/Drupal/aggregator/Tests/AggregatorRenderingTest.php
index 2f3097d..8d50401 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Tests/AggregatorRenderingTest.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Tests/AggregatorRenderingTest.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\aggregator\Tests;
 
+use Drupal\Component\Utility\String;
+
 /**
  * Tests rendering functionality in the Aggregator module.
  */
@@ -104,7 +106,7 @@ public function testFeedPage() {
     // Find the expected read_more link on the sources page.
     $href = 'aggregator/sources/' . $feed->id();
     $links = $this->xpath('//a[@href = :href]', array(':href' => url($href)));
-    $this->assert(isset($links[0]), format_string('Link to href %href found.', array('%href' => $href)));
+    $this->assertTrue(isset($links[0]), String::format('Link to href %href found.', array('%href' => $href)));
 
     // Check for the presence of a pager.
     $this->drupalGet('aggregator/sources/' . $feed->id());