diff --git a/src/Resource/AbstractResource.php b/src/Resource/AbstractResource.php
index 8a36b40..c3fd35c 100644
--- a/src/Resource/AbstractResource.php
+++ b/src/Resource/AbstractResource.php
@@ -173,7 +173,7 @@ public function clearUnsavedValues()
      * Gets the ID of the object.
      * This is a convenience method for getValue('id')
      *
-     * @return string       ID of this object, if available.
+     * @return string|null       ID of this object as string, or null if not available.
      */
     public function getID()
     {
diff --git a/test/ZammadAPIClient/GetIDTest.php b/test/ZammadAPIClient/GetIDTest.php
new file mode 100644
index 0000000..75db368
--- /dev/null
+++ b/test/ZammadAPIClient/GetIDTest.php
@@ -0,0 +1,65 @@
+<?php
+
+namespace ZammadAPIClient;
+
+use PHPUnit\Framework\TestCase;
+use ZammadAPIClient\Resource\AbstractResource;
+
+class GetIDTest extends TestCase
+{
+    private static $client;
+
+    public static function setUpBeforeClass(): void
+    {
+        self::$client = new Client([
+            'url'      => 'http://localhost:3000/',
+            'username' => 'test@example.com',
+            'password' => 'test',
+        ]);
+    }
+
+    public static function getClient()
+    {
+        return self::$client;
+    }
+
+    public function testGetIDBeforeSave()
+    {
+        $object = self::getClient()->resource( ResourceType::TICKET );
+
+        $this->assertNull(
+            $object->getID(),
+            'getID() must return null for unsaved object.'
+        );
+    }
+
+    public function testGetIDReturnsString()
+    {
+        $object = self::getClient()->resource( ResourceType::TICKET );
+        $object->setValue('id', 123);
+
+        $id = $object->getID();
+
+        $this->assertIsString(
+            $id,
+            'getID() must return a string.'
+        );
+
+        $this->assertSame(
+            '123',
+            $id,
+            'getID() must cast integer to string.'
+        );
+    }
+
+    public function testGetIDReturnsNullWhenIdNotSet()
+    {
+        $object = self::getClient()->resource( ResourceType::TICKET );
+        $object->setValue('title', 'test');
+
+        $this->assertNull(
+            $object->getID(),
+            'getID() must return null when id is not set.'
+        );
+    }
+}