diff --git a/CHANGELOG-2.1.md b/CHANGELOG-2.1.md
index 4cbefd922a..624c348c20 100644
--- a/CHANGELOG-2.1.md
+++ b/CHANGELOG-2.1.md
@@ -236,6 +236,7 @@ To get the diff between two versions, go to https://github.com/symfony/symfony/c
 
  * after login, the user is now redirected to `default_target_path` if `use_referer` is true and the referrer is the `login_path`.
  * added a way to remove a token from a session
+ * [BC BREAK] changed `MutableAclInterface::setParentAcl` to accept `null`, review your implementation to reflect this change.
 
 ### Serializer
 
diff --git a/src/Symfony/Component/Security/Acl/Domain/Acl.php b/src/Symfony/Component/Security/Acl/Domain/Acl.php
index 4a4a2e232b..a890bbec03 100644
--- a/src/Symfony/Component/Security/Acl/Domain/Acl.php
+++ b/src/Symfony/Component/Security/Acl/Domain/Acl.php
@@ -311,9 +311,9 @@ class Acl implements AuditableAclInterface, NotifyPropertyChanged
     /**
      * {@inheritDoc}
      */
-    public function setParentAcl(AclInterface $acl)
+    public function setParentAcl(AclInterface $acl = null)
     {
-        if (null === $acl->getId()) {
+        if (null !== $acl && null === $acl->getId()) {
             throw new \InvalidArgumentException('$acl must have an ID.');
         }
 
diff --git a/src/Symfony/Component/Security/Acl/Model/MutableAclInterface.php b/src/Symfony/Component/Security/Acl/Model/MutableAclInterface.php
index 521d307839..a846a9f161 100644
--- a/src/Symfony/Component/Security/Acl/Model/MutableAclInterface.php
+++ b/src/Symfony/Component/Security/Acl/Model/MutableAclInterface.php
@@ -114,9 +114,10 @@ interface MutableAclInterface extends AclInterface
     /**
      * Sets the parent ACL
      *
-     * @param AclInterface $acl
+     * @param AclInterface|null $acl
+     * @return void
      */
-    function setParentAcl(AclInterface $acl);
+    function setParentAcl(AclInterface $acl = null);
 
     /**
      * Updates a class-based ACE
diff --git a/tests/Symfony/Tests/Component/Security/Acl/Domain/AclTest.php b/tests/Symfony/Tests/Component/Security/Acl/Domain/AclTest.php
index 762ce7693d..90f13fb404 100644
--- a/tests/Symfony/Tests/Component/Security/Acl/Domain/AclTest.php
+++ b/tests/Symfony/Tests/Component/Security/Acl/Domain/AclTest.php
@@ -250,6 +250,9 @@ class AclTest extends \PHPUnit_Framework_TestCase
         $this->assertNull($acl->getParentAcl());
         $acl->setParentAcl($parentAcl);
         $this->assertSame($parentAcl, $acl->getParentAcl());
+
+        $acl->setParentAcl(null);
+        $this->assertNull($acl->getParentAcl());
     }
 
     public function testSetIsEntriesInheriting()