private function validateRole($role, $roles, $checkExistence = true)
    {
        if (empty($roles)) {
            return null;
        }

        $roleExists = array_key_exists($role, $roles);
        $condition =  $checkExistence ? !$roleExists : $roleExists;
        if ($condition) {
            $errorMessage = $checkExistence ? "Role %s doesn't exist" : 'Role %s already exists';
            throw new \InvalidArgumentException(sprintf($errorMessage, $role));
        }

        return $role;
    }