使用官方证书和回调报文解密示例，解析微信平台证书报文报java.lang.IllegalArgumentException: Illegal base64 character 22

//创建json解析器
JsonParser parse = new JsonParser();
JsonObject json = (JsonObject) parse.parse(pcs);
//json字符串解析,获取result节点
JsonObject encryptCertificate = json.get("encrypt_certificate").getAsJsonObject();
//获取节点的数值
String associatedData = encryptCertificate.get("associated_data").toString();
String ciphertext = encryptCertificate.get("ciphertext").toString();
String wxnonce = encryptCertificate.get("nonce").toString();
这是解析获取到的报文数据，在return new String(cipher.doFinal(Base64.getDecoder().decode(ciphertext)), "utf-8");报java.lang.IllegalArgumentException: Illegal base64 character 22
请问这是什么原因啊