diff --git a/wp-includes/classes.php b/wp-includes/classes.php
index 73dc220963..c7d59fa7e9 100644
--- a/wp-includes/classes.php
+++ b/wp-includes/classes.php
@@ -275,10 +275,10 @@ class WP {
 
 			if ( !empty( $this->query_vars[$wpvar] ) ) {
 				$this->query_vars[$wpvar] = (string) $this->query_vars[$wpvar];
-				if ( in_array( $wpvar, $taxonomy_query_vars ) ) {
+				if ( isset( $taxonomy_query_vars[$wpvar] ) ) {
 					$this->query_vars['taxonomy'] = $taxonomy_query_vars[$wpvar];
 					$this->query_vars['term'] = $this->query_vars[$wpvar];
-				} elseif ( in_array( $wpvar, $post_type_query_vars ) ) {
+				} elseif ( isset($post_type_query_vars[$wpvar] ) ) {
 					$this->query_vars['post_type'] = $post_type_query_vars[$wpvar];
 					$this->query_vars['name'] = $this->query_vars[$wpvar];
 				}