diff --git a/src/wp-includes/class-wp-customize-setting.php b/src/wp-includes/class-wp-customize-setting.php
index e9ca5bf348..fa943464a1 100644
--- a/src/wp-includes/class-wp-customize-setting.php
+++ b/src/wp-includes/class-wp-customize-setting.php
@@ -1103,7 +1103,7 @@ class WP_Customize_Nav_Menu_Item_Setting extends WP_Customize_Setting {
 			$i = 1;
 
 			foreach ( $items as $k => $item ) {
-				$items[ $k ]->$args['output_key'] = $i++;
+				$items[ $k ]->{$args['output_key']} = $i++;
 			}
 		}
 
diff --git a/src/wp-includes/nav-menu.php b/src/wp-includes/nav-menu.php
index 9061acedd1..d12abc55d9 100644
--- a/src/wp-includes/nav-menu.php
+++ b/src/wp-includes/nav-menu.php
@@ -638,7 +638,7 @@ function wp_get_nav_menu_items( $menu, $args = array() ) {
 		usort($items, '_sort_nav_menu_items');
 		$i = 1;
 		foreach( $items as $k => $item ) {
-			$items[$k]->$args['output_key'] = $i++;
+			$items[$k]->{$args['output_key']} = $i++;
 		}
 	}