Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 12543)
+++ wp-includes/formatting.php	(working copy)
@@ -2466,6 +2466,7 @@
 		case 'comments_per_page':
 		case 'thread_comments_depth':
 		case 'users_can_register':
+		case 'start_of_week':
 			$value = absint( $value );
 			break;