Index: src/wp-includes/class-wp-customize-setting.php
===================================================================
--- src/wp-includes/class-wp-customize-setting.php	(revision 33425)
+++ src/wp-includes/class-wp-customize-setting.php	(working copy)
@@ -1103,7 +1103,7 @@
 			$i = 1;
 
 			foreach ( $items as $k => $item ) {
-				$items[ $k ]->$args['output_key'] = $i++;
+				$items[ $k ]->{$args['output_key']} = $i++;
 			}
 		}
 
Index: src/wp-includes/nav-menu.php
===================================================================
--- src/wp-includes/nav-menu.php	(revision 33425)
+++ src/wp-includes/nav-menu.php	(working copy)
@@ -638,7 +638,7 @@
 		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++;
 		}
 	}