diff --git a/src/Resource/AbstractResource.php b/src/Resource/AbstractResource.php
index 5704c5c..8a36b40 100644
--- a/src/Resource/AbstractResource.php
+++ b/src/Resource/AbstractResource.php
@@ -177,7 +177,8 @@ public function clearUnsavedValues()
      */
     public function getID()
     {
-        return $this->getValue('id');
+        $value = $this->getValue('id');
+        return $value === null ? null : (string) $value;
     }
 
     /**