Index: wp-includes/rewrite.php
===================================================================
--- wp-includes/rewrite.php	(revision 15760)
+++ wp-includes/rewrite.php	(working copy)
@@ -337,7 +337,7 @@
 
 			// Do the query
 			$query = new WP_Query($query);
-			if ( $query->is_single || $query->is_page )
+			if ( is_object ( $query->post ) && ( $query->is_single || $query->is_page ) )
 				return $query->post->ID;
 			else
 				return 0;