diff --git a/src/Relation/BulkLoader.php b/src/Relation/BulkLoader.php
index fc2e8efa0..fd5924565 100644
--- a/src/Relation/BulkLoader.php
+++ b/src/Relation/BulkLoader.php
@@ -126,7 +126,11 @@ private function indexEntity(Node $node, array $keys, array $data, object $entit
         $pool = &$this->index;
         foreach ($keys as $k) {
             $keyValue = $data[$k] ?? throw new \LogicException("Bulk loader cannot get the value for the key `$k`.");
-            \is_scalar($keyValue) or throw new \InvalidArgumentException("Invalid value on the key `$k`.");
+            \is_scalar($keyValue) or $keyValue instanceof \Stringable
+                ? ($keyValue = (string) $keyValue)
+                : throw new \InvalidArgumentException(
+                    "Invalid value on the key `$k`. Expected scalar, got " . \get_debug_type($keyValue) . ".",
+                );
 
             \array_key_exists($keyValue, $pool) or $pool[$keyValue] = [];
             $pool = &$pool[$keyValue];
@@ -148,7 +152,7 @@ private function getEntity(array $pk, array $data): array
     {
         $result = $this->index;
         foreach ($pk as $k) {
-            $result = $result[$data[$k]] ?? throw new \LogicException('Cannot find indexed entity.');
+            $result = $result[(string) $data[$k]] ?? throw new \LogicException('Cannot find indexed entity.');
         }
 
         return $result;
diff --git a/tests/ORM/Fixtures/OneWayUuidTypecast.php b/tests/ORM/Fixtures/OneWayUuidTypecast.php
new file mode 100644
index 000000000..a6489d995
--- /dev/null
+++ b/tests/ORM/Fixtures/OneWayUuidTypecast.php
@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Cycle\ORM\Tests\Fixtures;
+
+use Cycle\ORM\Parser\CastableInterface;
+
+/**
+ * UUID typecast that only casts (string -> object) but does NOT uncast (object -> string).
+ * Simulates a real-world scenario where a custom typecast handler only implements CastableInterface.
+ */
+class OneWayUuidTypecast implements CastableInterface
+{
+    /** @var non-empty-string[] */
+    private array $rules = [];
+
+    public function setRules(array $rules): array
+    {
+        foreach ($rules as $key => $rule) {
+            if ($rule === 'uuid') {
+                unset($rules[$key]);
+                $this->rules[$key] = $rule;
+            }
+        }
+
+        return $rules;
+    }
+
+    public function cast(array $data): array
+    {
+        foreach ($this->rules as $column => $rule) {
+            if (!isset($data[$column])) {
+                continue;
+            }
+
+            $data[$column] = new UuidPrimaryKey((string) $data[$column]);
+        }
+
+        return $data;
+    }
+}
diff --git a/tests/ORM/Unit/Relation/BulkLoaderTest.php b/tests/ORM/Unit/Relation/BulkLoaderTest.php
index ece5312a2..22e08b5e5 100644
--- a/tests/ORM/Unit/Relation/BulkLoaderTest.php
+++ b/tests/ORM/Unit/Relation/BulkLoaderTest.php
@@ -5,13 +5,17 @@
 namespace Cycle\ORM\Tests\Unit\Relation;
 
 use Cycle\ORM\Factory;
+use Cycle\ORM\Heap\Node;
 use Cycle\ORM\Mapper\Mapper;
 use Cycle\ORM\ORM;
+use Cycle\ORM\Relation;
 use Cycle\ORM\Relation\BulkLoader;
 use Cycle\ORM\Relation\RelationLoaderInterface;
 use Cycle\ORM\Schema;
-use Cycle\ORM\Tests\Fixtures\User;
+use Cycle\ORM\Tests\Fixtures\OneWayUuidTypecast;
 use Cycle\ORM\Tests\Fixtures\Profile;
+use Cycle\ORM\Tests\Fixtures\User;
+use Cycle\ORM\Tests\Fixtures\UuidPrimaryKey;
 use PHPUnit\Framework\TestCase;
 
 class BulkLoaderTest extends TestCase
@@ -202,6 +206,74 @@ public function testLoadWithCustomOptions(): void
         $this->assertSame($loader, $result);
     }
 
+    /**
+     * BulkLoader should handle Stringable PK values when typecast is one-directional.
+     *
+     * Scenario: OneWayUuidTypecast implements only CastableInterface (cast: string→UuidPrimaryKey),
+     * but NOT UncastableInterface. So mapper->uncast() returns data with UuidPrimaryKey objects.
+     * BulkLoader::indexEntity() must handle Stringable objects instead of rejecting them via is_scalar().
+     */
+    public function testRunWithStringablePkAndOneWayTypecast(): void
+    {
+        $this->expectNotToPerformAssertions();
+
+        $schema = new Schema([
+            User::class => [
+                Schema::ROLE => 'user',
+                Schema::MAPPER => Mapper::class,
+                Schema::DATABASE => 'default',
+                Schema::TABLE => 'user',
+                Schema::PRIMARY_KEY => 'id',
+                Schema::COLUMNS => ['id', 'email', 'balance'],
+                Schema::TYPECAST => ['id' => 'uuid'],
+                Schema::TYPECAST_HANDLER => OneWayUuidTypecast::class,
+                Schema::SCHEMA => [],
+                Schema::RELATIONS => [
+                    'profile' => [
+                        Relation::TYPE => Relation::HAS_ONE,
+                        Relation::TARGET => Profile::class,
+                        Relation::SCHEMA => [
+                            Relation::CASCADE => true,
+                            Relation::INNER_KEY => 'id',
+                            Relation::OUTER_KEY => 'user_id',
+                        ],
+                    ],
+                ],
+            ],
+            Profile::class => [
+                Schema::ROLE => 'profile',
+                Schema::MAPPER => Mapper::class,
+                Schema::DATABASE => 'default',
+                Schema::TABLE => 'profile',
+                Schema::PRIMARY_KEY => 'id',
+                Schema::COLUMNS => ['id', 'user_id', 'image'],
+                Schema::SCHEMA => [],
+                Schema::RELATIONS => [],
+            ],
+        ]);
+
+        $orm = new ORM(
+            new Factory($this->createMock(\Cycle\Database\DatabaseProviderInterface::class)),
+            $schema,
+        );
+
+        $entity = new User();
+
+        // Simulate entity loaded from DB: typecast converted string PK to UuidPrimaryKey (Stringable),
+        // but uncast won't convert it back because OneWayUuidTypecast has no UncastableInterface.
+        $uuid = new UuidPrimaryKey('550e8400-e29b-41d4-a716-446655440001');
+        $node = new Node(Node::MANAGED, [
+            'id' => $uuid,
+            'email' => 'test@test.com',
+            'balance' => 100,
+        ], 'user');
+        $orm->getHeap()->attach($entity, $node);
+
+        $loader = (new BulkLoader($orm))->collect($entity);
+        $loader->load('profile');
+        $loader->run();
+    }
+
     private function createORM(): ORM
     {
         $schema = new Schema([