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 12afb44e25..dbcc40f0d0 100644
--- a/src/wp-admin/includes/class-wp-ajax-upgrader-skin.php
+++ b/src/wp-admin/includes/class-wp-ajax-upgrader-skin.php
@@ -95,9 +95,9 @@ class WP_Ajax_Upgrader_Skin extends Automatic_Upgrader_Skin {
 				}
 			}
 
-			// Count existing errors to generate an unique error code.
+			// Count existing errors to generate a unique error code.
 			$errors_count = count( $this->errors->get_error_codes() );
-			$this->errors->add( 'unknown_upgrade_error_' . $errors_count + 1, $string );
+			$this->errors->add( 'unknown_upgrade_error_' . ( $errors_count + 1 ), $string );
 		} elseif ( is_wp_error( $errors ) ) {
 			foreach ( $errors->get_error_codes() as $error_code ) {
 				$this->errors->add( $error_code, $errors->get_error_message( $error_code ), $errors->get_error_data( $error_code ) );