Index: webform.submissions.inc
===================================================================
RCS file: /cvs/drupal-contrib/contributions/modules/webform/includes/webform.submissions.inc,v
retrieving revision 1.30
diff -u -r1.30 webform.submissions.inc
--- webform.submissions.inc	17 Oct 2010 01:26:44 -0000	1.30
+++ webform.submissions.inc	20 Oct 2010 14:31:24 -0000
@@ -102,7 +102,7 @@
           'sid' => $submission->sid,
           'cid' => $cid,
           'no' => $delta,
-          'data' => $value
+          'data' => is_null($value) ? '' : $value,
         ))
         ->execute();
     }