diff --git a/wp-includes/class-wp-classic-to-block-menu-converter.php b/wp-includes/class-wp-classic-to-block-menu-converter.php
index 3aeb865302..6430aab6fa 100644
--- a/wp-includes/class-wp-classic-to-block-menu-converter.php
+++ b/wp-includes/class-wp-classic-to-block-menu-converter.php
@@ -20,7 +20,9 @@ class WP_Classic_To_Block_Menu_Converter {
 	 * @since 6.3.0
 	 *
 	 * @param WP_Term $menu The Menu term object of the menu to convert.
-	 * @return string the serialized and normalized parsed blocks.
+	 * @return string|WP_Error The serialized and normalized parsed blocks on success,
+	 *                         an empty string when there are no menus to convert,
+	 *                         or WP_Error on invalid menu.
 	 */
 	public static function convert( $menu ) {
 
@@ -34,7 +36,7 @@ class WP_Classic_To_Block_Menu_Converter {
 		$menu_items = wp_get_nav_menu_items( $menu->term_id, array( 'update_post_term_cache' => false ) );
 
 		if ( empty( $menu_items ) ) {
-			return array();
+			return '';
 		}
 
 		// Set up the $menu_item variables.
diff --git a/wp-includes/class-wp-navigation-fallback.php b/wp-includes/class-wp-navigation-fallback.php
index 70ddf11e4e..fb97c643bb 100644
--- a/wp-includes/class-wp-navigation-fallback.php
+++ b/wp-includes/class-wp-navigation-fallback.php
@@ -105,6 +105,10 @@ class WP_Navigation_Fallback {
 		// If there is a classic menu then convert it to blocks.
 		$classic_nav_menu_blocks = WP_Classic_To_Block_Menu_Converter::convert( $classic_nav_menu );
 
+		if ( is_wp_error( $classic_nav_menu_blocks ) ) {
+			return $classic_nav_menu_blocks;
+		}
+
 		if ( empty( $classic_nav_menu_blocks ) ) {
 			return new WP_Error( 'cannot_convert_classic_menu', __( 'Unable to convert Classic Menu to blocks.' ) );
 		}
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 964f84b9dd..1993e99af9 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-alpha-56421';
+$wp_version = '6.4-alpha-56422';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.