带Xcode 9和AES解密的CryptoSwift

我使用的是Xcode 9.0和CryptoSwift(0.7.2)。我试图扩展字符串以解密AES128加密的字符串。我已经成功地将CryptoSwift添加到POD中,但我遇到了以下编译错误-我做错了什么?

无法构造“PKCS7”,因为它没有可访问的初始值设定项

以下是扩展名:

import Foundation
import CryptoSwift

extension String {

    // https://stackoverflow.com/questions/27072021/aes-encrypt-and-decrypt
    func aesDecrypt(key: String, iv: String) throws -> String {
        let data = Data(base64Encoded: self)!
        let decrypted = try! AES(key: key, iv: iv, blockMode: .CBC, padding: PKCS7()).decrypt([UInt8](data))
        let decryptedData = Data(decrypted)
        return String(bytes: decryptedData.bytes, encoding: .utf8) ?? "Could not decrypt"
    }

}