diff --git a/src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php b/src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php
index 31bf55c72dd99..90809bbc2629b 100644
--- a/src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php
+++ b/src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php
@@ -264,6 +264,10 @@ public function clear()
     public function registerBag(SessionBagInterface $bag)
     {
         $this->bags[$bag->getName()] = $bag;
+
+        if ($this->isStarted()) {
+            $this->loadSessionBag($bag);
+        }
     }
 
     /**
@@ -422,12 +426,28 @@ protected function loadSession(array &$session = null)
         $bags = array_merge($this->bags, array($this->metadataBag));
 
         foreach ($bags as $bag) {
-            $key = $bag->getStorageKey();
-            $session[$key] = isset($session[$key]) ? $session[$key] : array();
-            $bag->initialize($session[$key]);
+            $this->loadSessionBag($bag, $session);
         }
 
         $this->started = true;
         $this->closed = false;
     }
+
+    /**
+     * Load and initialize the session for the given bag and pass the
+     * session variable into the bag.
+     *
+     * @param SessionBagInterface $bag
+     * @param array|null          $session
+     */
+    private function loadSessionBag(SessionBagInterface $bag, array &$session = null)
+    {
+        if (null === $session) {
+            $session = &$_SESSION;
+        }
+
+        $key = $bag->getStorageKey();
+        $session[$key] = isset($session[$key]) ? $session[$key] : array();
+        $bag->initialize($session[$key]);
+    }
 }
diff --git a/src/Symfony/Component/HttpFoundation/Tests/Session/Storage/NativeSessionStorageTest.php b/src/Symfony/Component/HttpFoundation/Tests/Session/Storage/NativeSessionStorageTest.php
index 4870115caa64c..42974c94a0f0b 100644
--- a/src/Symfony/Component/HttpFoundation/Tests/Session/Storage/NativeSessionStorageTest.php
+++ b/src/Symfony/Component/HttpFoundation/Tests/Session/Storage/NativeSessionStorageTest.php
@@ -254,4 +254,34 @@ public function testCanStartOutside54()
         $this->assertFalse(isset($_SESSION[$key]));
         $storage->start();
     }
+
+    public function testRegisterBagBeforeStarting()
+    {
+        $storage = $this->getStorage();
+        $bag = new AttributeBag('test_bag');
+        $bag->setName('test_bag');
+        $storage->registerBag($bag);
+
+        $storage->start();
+
+        // variable is set after session start, as the session handler overwrites $_SESSION
+        $_SESSION['test_bag']['some_key'] = 'some_value';
+
+        $this->assertSame('some_value', $bag->get('some_key'));
+    }
+
+    public function testRegisterBagAfterStarting()
+    {
+        $storage = $this->getStorage();
+        $storage->start();
+
+        // variable is set after session start, as the session handler overwrites $_SESSION
+        $_SESSION['test_bag']['some_key'] = 'some_value';
+
+        $bag = new AttributeBag('test_bag');
+        $bag->setName('test_bag');
+        $storage->registerBag($bag);
+
+        $this->assertSame('some_value', $bag->get('some_key'));
+    }
 }