diff --git a/src/Symfony/Component/Validator/Constraints/Composite.php b/src/Symfony/Component/Validator/Constraints/Composite.php
index bd7030ee27a4..b5cf5376764c 100644
--- a/src/Symfony/Component/Validator/Constraints/Composite.php
+++ b/src/Symfony/Component/Validator/Constraints/Composite.php
@@ -83,6 +83,13 @@ public function __construct($options = null)
             $mergedGroups = [];
 
             foreach ($nestedConstraints as $constraint) {
+                if ($constraint instanceof self && !$constraint->groups) {
+                    // this nested composite constraint is initialized without groups
+                    // when it happens, this one and its nested constraints needs to be
+                    // implicitly added in the default group
+                    $constraint->groups = [self::DEFAULT_GROUP];
+                }
+
                 foreach ($constraint->groups as $group) {
                     $mergedGroups[$group] = true;
                 }
diff --git a/src/Symfony/Component/Validator/Tests/Constraints/CollectionTest.php b/src/Symfony/Component/Validator/Tests/Constraints/CollectionTest.php
index fef129cfa749..1fb86fa669d9 100644
--- a/src/Symfony/Component/Validator/Tests/Constraints/CollectionTest.php
+++ b/src/Symfony/Component/Validator/Tests/Constraints/CollectionTest.php
@@ -14,6 +14,8 @@
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Validator\Constraints\Collection;
 use Symfony\Component\Validator\Constraints\Email;
+use Symfony\Component\Validator\Constraints\NotBlank;
+use Symfony\Component\Validator\Constraints\NotNull;
 use Symfony\Component\Validator\Constraints\Optional;
 use Symfony\Component\Validator\Constraints\Required;
 use Symfony\Component\Validator\Constraints\Valid;
@@ -100,4 +102,16 @@ public function testAcceptRequiredConstraintAsOneElementArray()
 
         $this->assertEquals($collection1, $collection2);
     }
+
+    public function testConstraintHasDefaultGroupWithOptionalValues()
+    {
+        $constraint = new Collection([
+            'foo' => new Required(),
+            'bar' => new Optional(),
+        ]);
+
+        $this->assertEquals(['Default'], $constraint->groups);
+        $this->assertEquals(['Default'], $constraint->fields['foo']->groups);
+        $this->assertEquals(['Default'], $constraint->fields['bar']->groups);
+    }
 }
diff --git a/src/Symfony/Component/Validator/Tests/Constraints/CollectionValidatorTest.php b/src/Symfony/Component/Validator/Tests/Constraints/CollectionValidatorTest.php
index e0ccdba754b0..11913eb2307a 100644
--- a/src/Symfony/Component/Validator/Tests/Constraints/CollectionValidatorTest.php
+++ b/src/Symfony/Component/Validator/Tests/Constraints/CollectionValidatorTest.php
@@ -143,6 +143,29 @@ public function testExtraFieldsDisallowed()
             ->assertRaised();
     }
 
+    public function testExtraFieldsDisallowedWithOptionalValues()
+    {
+        $constraint = new Optional();
+
+        $data = $this->prepareTestData([
+            'baz' => 6,
+        ]);
+
+        $this->validator->validate($data, new Collection([
+            'fields' => [
+                'foo' => $constraint,
+            ],
+            'extraFieldsMessage' => 'myMessage',
+        ]));
+
+        $this->buildViolation('myMessage')
+            ->setParameter('{{ field }}', '"baz"')
+            ->atPath('property.path[baz]')
+            ->setInvalidValue(6)
+            ->setCode(Collection::NO_SUCH_FIELD_ERROR)
+            ->assertRaised();
+    }
+
     // bug fix
     public function testNullNotConsideredExtraField()
     {
diff --git a/src/Symfony/Component/Validator/Tests/Constraints/CompositeTest.php b/src/Symfony/Component/Validator/Tests/Constraints/CompositeTest.php
index 2d42807821bb..3dd14e4bba2b 100644
--- a/src/Symfony/Component/Validator/Tests/Constraints/CompositeTest.php
+++ b/src/Symfony/Component/Validator/Tests/Constraints/CompositeTest.php
@@ -15,11 +15,12 @@
 use Symfony\Component\Validator\Constraints\Composite;
 use Symfony\Component\Validator\Constraints\NotBlank;
 use Symfony\Component\Validator\Constraints\NotNull;
+use Symfony\Component\Validator\Constraints\Optional;
 use Symfony\Component\Validator\Constraints\Valid;
 
 class ConcreteComposite extends Composite
 {
-    public $constraints;
+    public $constraints = [];
 
     protected function getCompositeOption()
     {
@@ -37,6 +38,30 @@ public function getDefaultOption()
  */
 class CompositeTest extends TestCase
 {
+    public function testConstraintHasDefaultGroup()
+    {
+        $constraint = new ConcreteComposite([
+            new NotNull(),
+            new NotBlank(),
+        ]);
+
+        $this->assertEquals(['Default'], $constraint->groups);
+        $this->assertEquals(['Default'], $constraint->constraints[0]->groups);
+        $this->assertEquals(['Default'], $constraint->constraints[1]->groups);
+    }
+
+    public function testNestedCompositeConstraintHasDefaultGroup()
+    {
+        $constraint = new ConcreteComposite([
+            new ConcreteComposite(),
+            new ConcreteComposite(),
+        ]);
+
+        $this->assertEquals(['Default'], $constraint->groups);
+        $this->assertEquals(['Default'], $constraint->constraints[0]->groups);
+        $this->assertEquals(['Default'], $constraint->constraints[1]->groups);
+    }
+
     public function testMergeNestedGroupsIfNoExplicitParentGroup()
     {
         $constraint = new ConcreteComposite([