Index: wp-includes/classes.php
===================================================================
--- wp-includes/classes.php	(revision 5043)
+++ wp-includes/classes.php	(working copy)
@@ -149,7 +149,8 @@
 			elseif (!empty($perma_query_vars[$wpvar]))
 				$this->query_vars[$wpvar] = $perma_query_vars[$wpvar];
 
-			$this->query_vars[$wpvar] = (string) $this->query_vars[$wpvar];
+			if( empty( $this->query_vars[$wpvar] ) == false )
+				$this->query_vars[$wpvar] = (string) $this->query_vars[$wpvar];
 		}
 
 		foreach ($this->private_query_vars as $var) {