解码base 64 swift 2.3

Question

我有以下扩展名：

extension String {

    func fromBase64() -> String? {
        guard let data = NSData(base64EncodedString: self, options: NSDataBase64DecodingOptions(rawValue: 0)) else {
            return nil
        }

        return String(data: data, encoding: NSUTF8StringEncoding)!
    }

    func toBase64() -> String? {
        guard let data = self.dataUsingEncoding(NSUTF8StringEncoding) else {
            return nil
        }

        return data.base64EncodedStringWithOptions(NSDataBase64EncodingOptions(rawValue: 0))
    }
}

我有一个64位编码字符串：R79gQDNTt / 0 + cjU7pduqfA ==

func doStuff(sample : String){

    let sampleBase64Decoded = sample.fromBase64()
}

调用此函数时，（doStuff（＆＃34; R79gQDNTt / 0 + cjU7pduqfA ==＆＃34;））在解开可选项时，我发现运行时错误，指的是找到nil：

return String(data: data, encoding: NSUTF8StringEncoding)!

Swift无法将base64数据转换为字符串的原因是什么？