public function assertIntOrFalse($config, $key, $lowest = null, $highest = null)
    {
        try {
            $this->assertInteger($config, $key, $lowest, $highest);
        } catch (ConfigurationException $e) {
            if ($config[$key]!==false) {
                throw new ConfigurationException(
                    "Key `{$key}` can be either false or a non-negative integer."
                );
            }
        }
        return $this;
    }