diff --git a/core/tests/Drupal/KernelTests/Core/Theme/FrontMatterTest.php b/core/tests/Drupal/KernelTests/Core/Theme/FrontMatterTest.php
index f79ce9d4ad..0f857dba98 100644
--- a/core/tests/Drupal/KernelTests/Core/Theme/FrontMatterTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Theme/FrontMatterTest.php
@@ -6,6 +6,7 @@
 use Drupal\KernelTests\KernelTestBase;
 use Drupal\Tests\Component\FrontMatter\FrontMatterTest as ComponentFrontMatterTest;
 use Symfony\Component\DependencyInjection\Definition;
+use Twig\Error\Error;
 use Twig\Error\SyntaxError;
 
 /**
@@ -56,7 +57,7 @@ public function register(ContainerBuilder $container) {
    * @return string
    *   The absolute path to the temporary file.
    */
-  protected function createTwigTemplate($content = '') {
+  protected function createTwigTemplate(string $content = ''): string {
     $file = tempnam(sys_get_temp_dir(), 'twig') . ".html.twig";
     file_put_contents($file, $content);
     return $file;
@@ -113,7 +114,7 @@ public function testFrontMatter($yaml, $line, $content = ComponentFrontMatterTes
     try {
       $this->twig->load($name);
     }
-    catch (\Twig_Error $error) {
+    catch (Error $error) {
       $this->assertEquals($line, $error->getTemplateLine());
     }
 
@@ -121,7 +122,7 @@ public function testFrontMatter($yaml, $line, $content = ComponentFrontMatterTes
     try {
       $this->twig->createTemplate($source)->render();
     }
-    catch (\Twig_Error $error) {
+    catch (Error $error) {
       $this->assertEquals($line, $error->getTemplateLine());
     }
   }
diff --git a/core/tests/Drupal/Tests/Component/FrontMatter/FrontMatterTest.php b/core/tests/Drupal/Tests/Component/FrontMatter/FrontMatterTest.php
index 090d5914c0..03bde3f57e 100644
--- a/core/tests/Drupal/Tests/Component/FrontMatter/FrontMatterTest.php
+++ b/core/tests/Drupal/Tests/Component/FrontMatter/FrontMatterTest.php
@@ -24,7 +24,7 @@ class FrontMatterTest extends TestCase {
   /**
    * Creates a front matter source string.
    *
-   * @param array|null $yaml
+   * @param array $yaml
    *   The YAML array to prepend as a front matter block.
    * @param string $content
    *   The source contents.
@@ -32,7 +32,7 @@ class FrontMatterTest extends TestCase {
    * @return string
    *   The new source.
    */
-  public static function createFrontMatterSource($yaml = [], $content = self::SOURCE) {
+  public static function createFrontMatterSource(array $yaml = [], string $content = self::SOURCE): string {
     // Encode YAML and wrap in a front matter block.
     $frontMatter = '';
     if (is_array($yaml)) {
@@ -49,7 +49,7 @@ public static function createFrontMatterSource($yaml = [], $content = self::SOUR
    * @covers ::create
    */
   public function testFrontMatterSerializerException() {
-    $this->expectException('AssertionError');
+    $this->expectException(\AssertionError::class);
     $this->expectExceptionMessage('The $serializer parameter must reference a class that implements Drupal\Component\Serialization\SerializationInterface.');
     FrontMatter::create('', '');
   }