diff --git a/src/Util/FormBuilderIterator.php b/src/Util/FormBuilderIterator.php
index a16083a2cd2..3c97587bc53 100644
--- a/src/Util/FormBuilderIterator.php
+++ b/src/Util/FormBuilderIterator.php
@@ -52,10 +52,20 @@ class FormBuilderIterator extends \RecursiveArrayIterator
      *
      * @param string|false $prefix
      */
-    public function __construct(FormBuilderInterface $formBuilder, $prefix = false)
+    public function __construct(FormBuilderInterface $formBuilder, $prefix = null)
     {
         parent::__construct();
         $this->formBuilder = $formBuilder;
+
+        // NEXT_MAJOR: Remove this block.
+        if (null !== $prefix && !\is_string($prefix)) {
+            @trigger_error(sprintf(
+                'Passing other type than string or null as argument 2 for method %s() is deprecated since'
+                .' sonata-project/admin-bundle 3.x. It will accept only string and null in version 4.0.',
+                __METHOD__
+            ), E_USER_DEPRECATED);
+        }
+
         // NEXT_MAJOR: Remove next line.
         $this->prefix = \is_string($prefix) ? $prefix : $formBuilder->getName();
         // NEXT_MAJOR: Uncomment next line.
diff --git a/tests/Util/FormBuilderIteratorTest.php b/tests/Util/FormBuilderIteratorTest.php
index ff15f7708cc..6ff9a8c6174 100644
--- a/tests/Util/FormBuilderIteratorTest.php
+++ b/tests/Util/FormBuilderIteratorTest.php
@@ -15,6 +15,7 @@
 
 use PHPUnit\Framework\TestCase;
 use Sonata\AdminBundle\Util\FormBuilderIterator;
+use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Symfony\Component\Form\Extension\Core\Type\TextType;
 use Symfony\Component\Form\FormBuilder;
@@ -25,6 +26,8 @@
  */
 class FormBuilderIteratorTest extends TestCase
 {
+    use ExpectDeprecationTrait;
+
     /**
      * @var EventDispatcherInterface
      */
@@ -69,4 +72,19 @@ public function testHasChildren(): void
         $iterator = new FormBuilderIterator($this->builder);
         $this->assertTrue($iterator->hasChildren());
     }
+
+    /**
+     * NEXT_MAJOR: Remove this test.
+     *
+     * @group legacy
+     */
+    public function testTriggersADeprecationWithWrongPrefixType(): void
+    {
+        $this->expectDeprecation('Passing other type than string or null as argument 2 for method Sonata\AdminBundle\Util\FormBuilderIterator::__construct() is deprecated since sonata-project/admin-bundle 3.x. It will accept only string and null in version 4.0.');
+
+        $this->builder->add('name', TextType::class);
+        $iterator = new FormBuilderIterator($this->builder, new \stdClass());
+
+        $this->assertSame($iterator->key(), 'name_name');
+    }
 }