diff --git a/src/wp-includes/class-wp-customize-nav-menus.php b/src/wp-includes/class-wp-customize-nav-menus.php
index 7fbbda8b4a..eec35ee7c6 100644
--- a/src/wp-includes/class-wp-customize-nav-menus.php
+++ b/src/wp-includes/class-wp-customize-nav-menus.php
@@ -753,17 +753,17 @@ final class WP_Customize_Nav_Menus {
 		$args['instance_number'] = $this->preview_nav_menu_instance_number;
 
 		$can_partial_refresh = (
-			$args['echo']
+			! empty( $args['echo'] )
 			&&
-			is_string( $args['fallback_cb'] )
+			( empty( $args['fallback_cb'] ) || is_string( $args['fallback_cb'] ) )
 			&&
-			is_string( $args['walker'] )
+			( empty( $args['walker'] ) || is_string( $args['walker'] ) )
 		);
 		$args['can_partial_refresh'] = $can_partial_refresh;
 
 		if ( ! $can_partial_refresh ) {
-			unset( $args['fallback_cb'] );
-			unset( $args['walker'] );
+			$args['fallback_cb'] = '';
+			$args['walker'] = '';
 		}
 
 		ksort( $args );
diff --git a/src/wp-includes/nav-menu-template.php b/src/wp-includes/nav-menu-template.php
index 2bd37a106d..3a4192c762 100644
--- a/src/wp-includes/nav-menu-template.php
+++ b/src/wp-includes/nav-menu-template.php
@@ -308,7 +308,7 @@ function wp_nav_menu( $args = array() ) {
 	 *  - Otherwise, bail.
 	 */
 	if ( ( !$menu || is_wp_error($menu) || ( isset($menu_items) && empty($menu_items) && !$args->theme_location ) )
-		&& $args->fallback_cb && is_callable( $args->fallback_cb ) )
+		&& isset( $args->fallback_cb ) && $args->fallback_cb && is_callable( $args->fallback_cb ) )
 			return call_user_func( $args->fallback_cb, (array) $args );
 
 	if ( ! $menu || is_wp_error( $menu ) )