diff --git a/wp-includes/customize/class-wp-customize-nav-menu-item-setting.php b/wp-includes/customize/class-wp-customize-nav-menu-item-setting.php
index 5da6515f5f..18cf83dfda 100644
--- a/wp-includes/customize/class-wp-customize-nav-menu-item-setting.php
+++ b/wp-includes/customize/class-wp-customize-nav-menu-item-setting.php
@@ -605,6 +605,11 @@ class WP_Customize_Nav_Menu_Item_Setting extends WP_Customize_Setting {
 			$item->post_title = $item->title;
 		}
 
+		// 'classes' should be an array, as in wp_setup_nav_menu_item().
+		if ( isset( $item->classes ) && is_scalar( $item->classes ) ) {
+			$item->classes = explode( ' ', $item->classes );
+		}
+
 		$item->ID    = $this->post_id;
 		$item->db_id = $this->post_id;
 		$post        = new WP_Post( (object) $item );
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 5faeceec9b..92307a5a2a 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-beta4-49623';
+$wp_version = '5.6-beta4-49624';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.