diff --git a/src/wp-includes/class-walker-nav-menu.php b/src/wp-includes/class-walker-nav-menu.php
index 22b6ae217b..871836f374 100644
--- a/src/wp-includes/class-walker-nav-menu.php
+++ b/src/wp-includes/class-walker-nav-menu.php
@@ -72,7 +72,7 @@ class Walker_Nav_Menu extends Walker {
 		$class_names = join( ' ', apply_filters( 'nav_menu_submenu_css_class', $classes, $args, $depth ) );
 		$class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';
 
-		$output .= "{$n}{$indent}<ul $class_names>{$n}";
+		$output .= "{$n}{$indent}<ul$class_names>{$n}";
 	}
 
 	/**