“ catch”块不可访问，因为“ do”块中未引发任何错误

Question

我正在编写带有错误处理的闭包，下面是我的代码，但是我收到一条警告：“'catch'块无法访问，因为在'do'块中未引发任何错误”

enum Result {
case success(r: EmailContentData)
case failure(e: Error)
}

static func getMessageList(index : Int, pageCount: Int, completion : @escaping (Result) -> ())
{
    DispatchQueue.global(qos: .userInteractive).async
        {
            let apiString : String = String(index) + "/" + String(pageCount)
            ApiHelper.sharedSession.post(Constant.API.messagelistApi + apiString, postData: NSDictionary(), methodtype: Constant.API.httpGet) { (isError, data, errorDescription) in
                guard let data = data else { return }
                do {
                    let result = EmailContentData.init(fromDictionary: data.value(forKey:"data") as! NSDictionary)
                    completion(Result.success(r: result))
                } catch let error {
                    completion(Result.failure(e: error ))
                }
            }
    }
}

如何删除此警告并在控制器中捕获错误？