diff --git a/wp-includes/classes.php b/wp-includes/classes.php
index 5bdeebe8ec..6bdfb2eff2 100644
--- a/wp-includes/classes.php
+++ b/wp-includes/classes.php
@@ -149,7 +149,8 @@ class WP {
 			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] ) )
+				$this->query_vars[$wpvar] = (string) $this->query_vars[$wpvar];
 		}
 
 		foreach ($this->private_query_vars as $var) {