private function getManagerForUser($user)
    {
        $tries = [];

        foreach ($this->managers as $manager) {
            if ($manager->supportsUser($user)) {
                return $manager;
            }

            $tries[] = get_class($manager);
        }

        throw new \RuntimeException(
            sprintf(
                'User "%s" is not supported by any UserManager. Tried "%s".',
                $user,
                implode('", "', $tries)
            )
        );
    }