public function setPrefix($prefix)
    {
        if (!is_string($prefix)) {
            throw new InvalidArgumentException(
                'Prefix must be a string'
            );
        }

        /** @see \Stash\Pool\::setNamespace */
        if (!ctype_alnum($prefix)) {
            throw new InvalidArgumentException(
                'Prefix must be alphanumeric'
            );
        }

        $this->prefix = $prefix;
        return $this;
    }