diff --git a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
index 2e455188ceec..a7d901356d90 100644
--- a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
+++ b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
@@ -241,6 +241,7 @@ public function merge(ContainerBuilder $container)
 
         $this->addDefinitions($container->getDefinitions());
         $this->addAliases($container->getAliases());
+        $this->addInterfaceInjectors($container->getInterfaceInjectors());
         $this->parameterBag->add($container->getParameterBag()->all());
 
         foreach ($container->getResources() as $resource) {
diff --git a/src/Symfony/Component/DependencyInjection/InterfaceInjector.php b/src/Symfony/Component/DependencyInjection/InterfaceInjector.php
index cbc27500c3f7..95a4db5d40c1 100755
--- a/src/Symfony/Component/DependencyInjection/InterfaceInjector.php
+++ b/src/Symfony/Component/DependencyInjection/InterfaceInjector.php
@@ -79,8 +79,11 @@ public function processDefinition(Definition $definition, $class = null)
     public function supports($object)
     {
         if (is_string($object)) {
-            $class = new \ReflectionClass($object);
-            $object = $class->newInstance();
+            $reflection = new \ReflectionClass($object);
+
+            return $reflection->implementsInterface($this->class)
+                   || $reflection->isSubClassOf($this->class)
+                   || $object === $this->class;
         }
 
         if ( ! is_object($object)) {