diff --git a/src/Symfony/Component/Form/DependencyInjection/FormPass.php b/src/Symfony/Component/Form/DependencyInjection/FormPass.php
index 358bf71205..f3cb08577e 100644
--- a/src/Symfony/Component/Form/DependencyInjection/FormPass.php
+++ b/src/Symfony/Component/Form/DependencyInjection/FormPass.php
@@ -92,18 +92,20 @@ class FormPass implements CompilerPassInterface
             $serviceDefinition = $container->getDefinition($serviceId);
 
             $tag = $serviceDefinition->getTag($this->formTypeExtensionTag);
+            $typeExtensionClass = $container->getParameterBag()->resolveValue($serviceDefinition->getClass());
+
             if (isset($tag[0]['extended_type'])) {
-                if (!method_exists($serviceDefinition->getClass(), 'getExtendedTypes')) {
-                    @trigger_error(sprintf('Not implementing the static getExtendedTypes() method in %s when implementing the %s is deprecated since Symfony 4.2. The method will be added to the interface in 5.0.', $serviceDefinition->getClass(), FormTypeExtensionInterface::class), E_USER_DEPRECATED);
+                if (!method_exists($typeExtensionClass, 'getExtendedTypes')) {
+                    @trigger_error(sprintf('Not implementing the static getExtendedTypes() method in %s when implementing the %s is deprecated since Symfony 4.2. The method will be added to the interface in 5.0.', $typeExtensionClass, FormTypeExtensionInterface::class), E_USER_DEPRECATED);
                 }
 
                 $typeExtensions[$tag[0]['extended_type']][] = new Reference($serviceId);
-                $typeExtensionsClasses[] = $serviceDefinition->getClass();
-            } elseif (method_exists($serviceDefinition->getClass(), 'getExtendedTypes')) {
+                $typeExtensionsClasses[] = $typeExtensionClass;
+            } elseif (method_exists($typeExtensionClass, 'getExtendedTypes')) {
                 $extendsTypes = false;
 
-                $typeExtensionsClasses[] = $serviceDefinition->getClass();
-                foreach ($serviceDefinition->getClass()::getExtendedTypes() as $extendedType) {
+                $typeExtensionsClasses[] = $typeExtensionClass;
+                foreach ($typeExtensionClass::getExtendedTypes() as $extendedType) {
                     $typeExtensions[$extendedType][] = new Reference($serviceId);
                     $extendsTypes = true;
                 }
@@ -112,7 +114,7 @@ class FormPass implements CompilerPassInterface
                     throw new InvalidArgumentException(sprintf('The getExtendedTypes() method for service "%s" does not return any extended types.', $serviceId));
                 }
             } else {
-                throw new InvalidArgumentException(sprintf('"%s" tagged services have to implement the static getExtendedTypes() method. The class for service "%s" does not implement it.', $this->formTypeExtensionTag, $serviceId));
+                throw new InvalidArgumentException(sprintf('"%s" tagged services have to implement the static getExtendedTypes() method. Class "%s" for service "%s" does not implement it.', $this->formTypeExtensionTag, $typeExtensionClass, $serviceId));
             }
         }
 
diff --git a/src/Symfony/Component/Form/Tests/DependencyInjection/FormPassTest.php b/src/Symfony/Component/Form/Tests/DependencyInjection/FormPassTest.php
index 5d0518689a..28e1fde10f 100644
--- a/src/Symfony/Component/Form/Tests/DependencyInjection/FormPassTest.php
+++ b/src/Symfony/Component/Form/Tests/DependencyInjection/FormPassTest.php
@@ -97,10 +97,14 @@ class FormPassTest extends TestCase
     /**
      * @dataProvider addTaggedTypeExtensionsDataProvider
      */
-    public function testAddTaggedTypeExtensions(array $extensions, array $expectedRegisteredExtensions)
+    public function testAddTaggedTypeExtensions(array $extensions, array $expectedRegisteredExtensions, array $parameters = [])
     {
         $container = $this->createContainerBuilder();
 
+        foreach ($parameters as $name => $value) {
+            $container->setParameter($name, $value);
+        }
+
         $container->setDefinition('form.extension', $this->createExtensionDefinition());
 
         foreach ($extensions as $serviceId => $config) {
@@ -191,6 +195,27 @@ class FormPassTest extends TestCase
                     ]),
                 ],
             ],
+            [
+                [
+                    'my.type_extension1' => [
+                        'class' => '%type1_extension_class%',
+                        'tag' => ['extended_type' => 'type1'],
+                    ],
+                    'my.type_extension2' => [
+                        'class' => '%type1_extension_class%',
+                        'tag' => [],
+                    ],
+                ],
+                [
+                    'type1' => new IteratorArgument([
+                        new Reference('my.type_extension1'),
+                        new Reference('my.type_extension2'),
+                    ]),
+                ],
+                [
+                    'type1_extension_class' => Type1TypeExtension::class,
+                ],
+            ],
         ];
     }
 
@@ -261,7 +286,7 @@ class FormPassTest extends TestCase
 
     /**
      * @expectedException \InvalidArgumentException
-     * @expectedExceptionMessage "form.type_extension" tagged services have to implement the static getExtendedTypes() method. The class for service "my.type_extension" does not implement it.
+     * @expectedExceptionMessage "form.type_extension" tagged services have to implement the static getExtendedTypes() method. Class "stdClass" for service "my.type_extension" does not implement it.
      */
     public function testAddTaggedFormTypeExtensionWithoutExtendedTypeAttributeNorImplementingGetExtendedTypes()
     {