diff --git a/src/Symfony/Component/DependencyInjection/Compiler/CheckDefinitionValidityPass.php b/src/Symfony/Component/DependencyInjection/Compiler/CheckDefinitionValidityPass.php
index 728332ceaf..c8978f377f 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/CheckDefinitionValidityPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/CheckDefinitionValidityPass.php
@@ -79,7 +79,7 @@ class CheckDefinitionValidityPass implements CompilerPassInterface
             foreach ($definition->getTags() as $name => $tags) {
                 foreach ($tags as $attributes) {
                     foreach ($attributes as $attribute => $value) {
-                        if (!is_scalar($value)) {
+                        if (!is_scalar($value) && null !== $value) {
                             throw new RuntimeException(sprintf('A "tags" attribute must be of a scalar-type for service "%s", tag "%s", attribute "%s".', $id, $name, $attribute));
                         }
                     }
diff --git a/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php b/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
index 21314b0b11..11e94c8c40 100644
--- a/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
+++ b/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php
@@ -224,7 +224,7 @@ class YamlFileLoader extends FileLoader
                 unset($tag['name']);
 
                 foreach ($tag as $attribute => $value) {
-                    if (!is_scalar($value)) {
+                    if (!is_scalar($value) && null !== $value) {
                         throw new InvalidArgumentException(sprintf('A "tags" attribute must be of a scalar-type for service "%s", tag "%s", attribute "%s" in %s.', $id, $name, $attribute, $file));
                     }
                 }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Compiler/CheckDefinitionValidityPassTest.php b/src/Symfony/Component/DependencyInjection/Tests/Compiler/CheckDefinitionValidityPassTest.php
index 2580318a08..b5e49842e7 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Compiler/CheckDefinitionValidityPassTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Compiler/CheckDefinitionValidityPassTest.php
@@ -61,6 +61,17 @@ class CheckDefinitionValidityPassTest extends \PHPUnit_Framework_TestCase
         $this->process($container);
     }
 
+    public function testValidTags()
+    {
+        $container = new ContainerBuilder();
+        $container->register('a', 'class')->addTag('foo', array('bar' => 'baz'));
+        $container->register('b', 'class')->addTag('foo', array('bar' => null));
+        $container->register('c', 'class')->addTag('foo', array('bar' => 1));
+        $container->register('d', 'class')->addTag('foo', array('bar' => 1.1));
+
+        $this->process($container);
+    }
+
     /**
      * @expectedException \Symfony\Component\DependencyInjection\Exception\RuntimeException
      */