无法将“MockURLSession”类型的值转换为预期的参数类型“MockURLSession？”错误

Question

我有一个函数我试图通过注入mockURLSession作为参数进行单元测试：

internal func getConnection(urlRequest: Request, mockUrlSession: MockURLSession?) {

    if mockUrlSession == nil {
        let dataTask = self.urlSession.dataTask(with: urlRequest as URLRequest)
         dataTask.resume()
    } else {

        let dataTask = mockUrlSession.dataTask(with: urlRequest, completionHandler: { (data, response, error) in

        })

        dataTask.resume()
    }

但是当我尝试进行单元测试时，我收到了错误：

  

无法将“MockURLSession”类型的值转换为预期参数   输入'MockURLSession？'

这是单元测试：

 func test_urlSession404(){
        let mockResponse = HTTPURLResponse(url: (self.urlRequest?.url!)!, statusCode: 404, httpVersion: "HTTP/1.1", headerFields: nil)!
        var mockSession = MockURLSession(mockResponse: mockResponse)
        sut?.getConnection(urlRequest: self.urlRequest!, mockUrlSession: mockSession)//error here
    }

即使我没有使MockURLSession参数可选，错误仍会显示减去？在MockURLSession结束时。我很困惑因为它们属于同一类型，为什么它不能转换，有没有办法让它可以注入mockURLSession？