diff --git a/src/wp-includes/class-wp-textdomain-registry.php b/src/wp-includes/class-wp-textdomain-registry.php
index 8ff8ad0ae2..54f5f15c4d 100644
--- a/src/wp-includes/class-wp-textdomain-registry.php
+++ b/src/wp-includes/class-wp-textdomain-registry.php
@@ -228,7 +228,11 @@ class WP_Textdomain_Registry {
 	 * @return void
 	 */
 	public function invalidate_mo_files_cache( $upgrader, $hook_extra ) {
-		if ( 'translation' !== $hook_extra['type'] || array() === $hook_extra['translations'] ) {
+		if (
+			! isset( $hook_extra['type'] ) ||
+			'translation' !== $hook_extra['type'] ||
+			array() === $hook_extra['translations']
+		) {
 			return;
 		}