diff --git a/src/wp-admin/includes/class-wp-ajax-upgrader-skin.php b/src/wp-admin/includes/class-wp-ajax-upgrader-skin.php
index 8698e20445..7d7f2bee11 100644
--- a/src/wp-admin/includes/class-wp-ajax-upgrader-skin.php
+++ b/src/wp-admin/includes/class-wp-ajax-upgrader-skin.php
@@ -99,7 +99,7 @@ class WP_Ajax_Upgrader_Skin extends Automatic_Upgrader_Skin {
 			}
 
 			// Count existing errors to generate an unique error code.
-			$errors_count = count( $errors->get_error_codes() );
+			$errors_count = count( $this->errors->get_error_codes() );
 			$this->errors->add( 'unknown_upgrade_error_' . $errors_count + 1 , $string );
 		} elseif ( is_wp_error( $errors ) ) {
 			foreach ( $errors->get_error_codes() as $error_code ) {