diff --git a/src/Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.php b/src/Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.php
index a0ad77edd0796..607f1d1fe743d 100644
--- a/src/Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.php
+++ b/src/Symfony/Component/Config/Definition/Builder/ArrayNodeDefinition.php
@@ -226,7 +226,9 @@ public function canBeEnabled()
             ->beforeNormalization()
                 ->ifArray()
                 ->then(function ($v) {
-                    $v['enabled'] = isset($v['enabled']) ? $v['enabled'] : true;
+                    if (!isset($v['enabled'])) {
+                        $v['enabled'] = !empty($v);
+                    }
 
                     return $v;
                 })
diff --git a/src/Symfony/Component/Config/Tests/Definition/Builder/ArrayNodeDefinitionTest.php b/src/Symfony/Component/Config/Tests/Definition/Builder/ArrayNodeDefinitionTest.php
index 16a38c30b0f9d..4b6f4d7fd1013 100644
--- a/src/Symfony/Component/Config/Tests/Definition/Builder/ArrayNodeDefinitionTest.php
+++ b/src/Symfony/Component/Config/Tests/Definition/Builder/ArrayNodeDefinitionTest.php
@@ -207,6 +207,20 @@ public function testCanBeDisabled()
         $this->assertTrue($this->getField($enabledNode, 'defaultValue'));
     }
 
+    public function testEnableableNodeIsDisabledForEmptyConfigurationWhenNormalized()
+    {
+        $config = array();
+
+        $node = new ArrayNodeDefinition('root');
+        $node->canBeEnabled();
+
+        $this->assertEquals(
+            array('enabled' => false),
+            $node->getNode()->normalize($config),
+            'An enableable node is disabled by default'
+        );
+    }
+
     public function testIgnoreExtraKeys()
     {
         $node = new ArrayNodeDefinition('root');
@@ -240,6 +254,7 @@ public function getEnableableNodeFixtures()
             array(array('enabled' => true, 'foo' => 'baz'), array(array('foo' => 'baz')), 'any configuration enables an enableable node'),
             array(array('enabled' => false, 'foo' => 'baz'), array(array('foo' => 'baz', 'enabled' => false)), 'An enableable node can be disabled'),
             array(array('enabled' => false, 'foo' => 'bar'), array(false), 'false disables an enableable node'),
+            array(array('enabled' => false, 'foo' => 'bar'), array(), 'enableable node is disabled by default'),
         );
     }
 
diff --git a/src/Symfony/Component/Config/Tests/Definition/Builder/TreeBuilderTest.php b/src/Symfony/Component/Config/Tests/Definition/Builder/TreeBuilderTest.php
index 0a9312346e989..6e99469a4e48c 100644
--- a/src/Symfony/Component/Config/Tests/Definition/Builder/TreeBuilderTest.php
+++ b/src/Symfony/Component/Config/Tests/Definition/Builder/TreeBuilderTest.php
@@ -12,6 +12,7 @@
 namespace Symfony\Component\Config\Tests\Definition\Builder;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\Config\Definition\Processor;
 use Symfony\Component\Config\Tests\Fixtures\Builder\NodeBuilder as CustomNodeBuilder;
 use Symfony\Component\Config\Definition\Builder\TreeBuilder;
 
@@ -131,4 +132,22 @@ public function testDefinitionExampleGetsTransferredToNode()
         $this->assertInternalType('array', $tree->getExample());
         $this->assertEquals('example', $children['child']->getExample());
     }
+
+    public function testRootNodeThatCanBeEnabledIsDisabledByDefault()
+    {
+        $builder = new TreeBuilder();
+
+        $builder->root('test')
+            ->canBeEnabled();
+
+        $tree = $builder->buildTree();
+        $children = $tree->getChildren();
+
+        $this->assertFalse($children['enabled']->getDefaultValue());
+
+        $processor = new Processor();
+        $result = $processor->process($tree, array());
+
+        $this->assertEquals(array('enabled' => false), $result);
+    }
 }
diff --git a/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php b/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
index b6b9e93a86ca3..c548ed459c0c7 100644
--- a/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
+++ b/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
@@ -425,6 +425,10 @@ private function loadFromExtensions(array $content)
                 continue;
             }
 
+            if (null === $values) {
+                $values = array('enabled' => null);
+            }
+
             if (!is_array($values)) {
                 $values = array();
             }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/FooConfiguration.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/FooConfiguration.php
new file mode 100644
index 0000000000000..e37bef3e15640
--- /dev/null
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/FooConfiguration.php
@@ -0,0 +1,15 @@
+<?php
+
+use Symfony\Component\Config\Definition\Builder\TreeBuilder;
+
+final class FooConfiguration implements \Symfony\Component\Config\Definition\ConfigurationInterface
+{
+    public function getConfigTreeBuilder()
+    {
+        $builder = new TreeBuilder();
+        $rootNode = $builder->root('foo');
+        $rootNode->canBeEnabled();
+
+        return $builder;
+    }
+}
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/FooExtension.php b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/FooExtension.php
new file mode 100644
index 0000000000000..e4ff380416f38
--- /dev/null
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/includes/FooExtension.php
@@ -0,0 +1,19 @@
+<?php
+
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Extension\Extension;
+
+final class FooExtension extends Extension
+{
+    public function load(array $configs, ContainerBuilder $container)
+    {
+        $config = $this->processConfiguration(new \FooConfiguration(), $configs);
+
+        $container->setParameter('foo_extension_enabled', $config['enabled']);
+    }
+
+    public function getAlias()
+    {
+        return 'foo';
+    }
+}
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/extension_nullable.yml b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/extension_nullable.yml
new file mode 100644
index 0000000000000..b5051d957328d
--- /dev/null
+++ b/src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/extension_nullable.yml
@@ -0,0 +1 @@
+foo: ~
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php b/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php
index 85c74b572d080..fdc3ffab7e412 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Loader/YamlFileLoaderTest.php
@@ -31,6 +31,8 @@ public static function setUpBeforeClass()
         self::$fixturesPath = realpath(__DIR__.'/../Fixtures/');
         require_once self::$fixturesPath.'/includes/foo.php';
         require_once self::$fixturesPath.'/includes/ProjectExtension.php';
+        require_once self::$fixturesPath.'/includes/FooExtension.php';
+        require_once self::$fixturesPath.'/includes/FooConfiguration.php';
     }
 
     /**
@@ -207,6 +209,19 @@ public function testExtensions()
         }
     }
 
+    public function testNullableExtension()
+    {
+        $container = new ContainerBuilder();
+        $container->registerExtension(new \FooExtension());
+        $loader = new YamlFileLoader($container, new FileLocator(self::$fixturesPath.'/yaml'));
+        $loader->load('extension_nullable.yml');
+        $container->compile();
+
+        $isEnabled = $container->getParameter('foo_extension_enabled');
+
+        $this->assertTrue($isEnabled);
+    }
+
     public function testSupports()
     {
         $loader = new YamlFileLoader(new ContainerBuilder(), new FileLocator());