diff --git a/src/Symfony/Component/DependencyInjection/Container.php b/src/Symfony/Component/DependencyInjection/Container.php
index 3cfdd616c5..f6c953f550 100644
--- a/src/Symfony/Component/DependencyInjection/Container.php
+++ b/src/Symfony/Component/DependencyInjection/Container.php
@@ -223,6 +223,7 @@ class Container implements IntrospectableContainerInterface
             if ('service_container' === $id
                 || isset($this->aliases[$id])
                 || isset($this->services[$id])
+                || array_key_exists($id, $this->services)
             ) {
                 return true;
             }
@@ -265,7 +266,7 @@ class Container implements IntrospectableContainerInterface
                 $id = $this->aliases[$id];
             }
             // Re-use shared service instance if it exists.
-            if (isset($this->services[$id])) {
+            if (isset($this->services[$id]) || array_key_exists($id, $this->services)) {
                 return $this->services[$id];
             }
 
@@ -347,7 +348,7 @@ class Container implements IntrospectableContainerInterface
             $id = $this->aliases[$id];
         }
 
-        return isset($this->services[$id]);
+        return isset($this->services[$id]) || array_key_exists($id, $this->services);
     }
 
     /**
diff --git a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
index 70d1efda4c..0813110ded 100644
--- a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
+++ b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
@@ -436,7 +436,7 @@ class ContainerBuilder extends Container implements TaggedContainerInterface
             return $service;
         }
 
-        if (!isset($this->definitions[$id]) && isset($this->aliasDefinitions[$id])) {
+        if (!array_key_exists($id, $this->definitions) && isset($this->aliasDefinitions[$id])) {
             return $this->get($this->aliasDefinitions[$id]);
         }
 
@@ -784,7 +784,7 @@ class ContainerBuilder extends Container implements TaggedContainerInterface
      */
     public function hasDefinition($id)
     {
-        return isset($this->definitions[strtolower($id)]);
+        return array_key_exists(strtolower($id), $this->definitions);
     }
 
     /**
@@ -800,7 +800,7 @@ class ContainerBuilder extends Container implements TaggedContainerInterface
     {
         $id = strtolower($id);
 
-        if (!isset($this->definitions[$id])) {
+        if (!array_key_exists($id, $this->definitions)) {
             throw new ServiceNotFoundException($id);
         }