diff --git a/src/DataTransferObject.php b/src/DataTransferObject.php
index dc7a346..4d9df8f 100644
--- a/src/DataTransferObject.php
+++ b/src/DataTransferObject.php
@@ -103,7 +103,7 @@ protected function setMap(array $items): void
     protected function setItems(array $items): void
     {
         foreach ($items as $key => $value) {
-            $this->setValue($key, $value);
+            $this->setValue((string) $key, $value);
         }
     }
 
diff --git a/tests/Unit/IntKeyTest.php b/tests/Unit/IntKeyTest.php
new file mode 100644
index 0000000..2077098
--- /dev/null
+++ b/tests/Unit/IntKeyTest.php
@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit;
+
+use Tests\Fixtures\Map;
+use Tests\TestCase;
+
+class IntKeyTest extends TestCase
+{
+    public function testMake()
+    {
+        $object = Map::make([
+            $this->foo,
+            $this->bar,
+            $this->baz,
+        ]);
+
+        $this->assertNull($object->foo);
+        $this->assertNull($object->bar);
+        $this->assertNull($object->baz);
+    }
+}