diff --git a/src/DependencyInjection/Configuration.php b/src/DependencyInjection/Configuration.php
index 9475d63..5c69dd1 100644
--- a/src/DependencyInjection/Configuration.php
+++ b/src/DependencyInjection/Configuration.php
@@ -20,7 +20,7 @@
 
 class Configuration implements ConfigurationInterface
 {
-    public function getConfigTreeBuilder()
+    public function getConfigTreeBuilder(): TreeBuilder
     {
         $treeBuilder = new TreeBuilder('pimcore_elasticsearch_client');
 
diff --git a/src/DependencyInjection/PimcoreElasticsearchClientExtension.php b/src/DependencyInjection/PimcoreElasticsearchClientExtension.php
index f169a05..40b9111 100644
--- a/src/DependencyInjection/PimcoreElasticsearchClientExtension.php
+++ b/src/DependencyInjection/PimcoreElasticsearchClientExtension.php
@@ -32,7 +32,7 @@ class PimcoreElasticsearchClientExtension extends ConfigurableExtension implemen
 
     const PIMCORE_CLIENT_PREFIX = 'pimcore.elasticsearch.custom_client.';
 
-    protected function loadInternal(array $mergedConfig, ContainerBuilder $container)
+    protected function loadInternal(array $mergedConfig, ContainerBuilder $container): void
     {
         $loader = new YamlFileLoader($container, new FileLocator(__DIR__.'/../../config'));
         $loader->load('services.yaml');
@@ -55,7 +55,7 @@ protected function loadInternal(array $mergedConfig, ContainerBuilder $container
         $container->addDefinitions($definitions);
     }
 
-    public function prepend(ContainerBuilder $container)
+    public function prepend(ContainerBuilder $container): void
     {
         $loader = new YamlFileLoader($container, new FileLocator(__DIR__.'/../../config'));
         $loader->load('default_config.yaml');