diff --git a/composer.json b/composer.json
index dddff0ff..9e7c6598 100644
--- a/composer.json
+++ b/composer.json
@@ -14,14 +14,14 @@
     "require": {
         "php": "^7.1.3 || ^8.0.0",
         "ezyang/htmlpurifier": "~4.0",
-        "symfony/config": "~3.4 || ~4.0 || ^5.0",
-        "symfony/dependency-injection": "~3.4.1 || ^4.0.1 || ^5.0",
-        "symfony/http-kernel": "~3.4.1 || ^4.0.1 || ^5.0"
+        "symfony/config": "~3.4 || ~4.0 || ^5.0 || ^6.0",
+        "symfony/dependency-injection": "~3.4.1 || ^4.0.1 || ^5.0 || ^6.0",
+        "symfony/http-kernel": "~3.4.1 || ^4.0.1 || ^5.0 || ^6.0"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.0",
-        "symfony/form": "~3.4.1 || ^4.0.1 || ^5.0",
-        "symfony/phpunit-bridge": "4.4.*",
+        "symfony/form": "~3.4.1 || ^4.0.1 || ^5.0 || ^6.0",
+        "symfony/phpunit-bridge": "4.4.* || ^5.0 || ^6.0",
         "twig/twig": "^1.35.0 || ^2.4.4 || ^3.0"
     },
     "autoload": {
diff --git a/src/DependencyInjection/ExerciseHTMLPurifierExtension.php b/src/DependencyInjection/ExerciseHTMLPurifierExtension.php
index 76965aaf..48105196 100644
--- a/src/DependencyInjection/ExerciseHTMLPurifierExtension.php
+++ b/src/DependencyInjection/ExerciseHTMLPurifierExtension.php
@@ -87,7 +87,7 @@ public function load(array $configs, ContainerBuilder $container)
         ;
     }
 
-    public function getAlias()
+    public function getAlias(): string
     {
         return 'exercise_html_purifier';
     }