diff --git a/webform.module b/webform.module
index 2312e2a..e90febb 100644
--- a/webform.module
+++ b/webform.module
@@ -1942,7 +1942,7 @@
     );
     $form['details']['sid'] = array(
       '#type' => 'hidden',
-      '#value' => isset($submission->sid) ? $submission->sid : '',
+      '#value' => isset($submission->sid) ? $submission->sid : NULL,
     );
     $form['details']['uid'] = array(
       '#type' => 'value',