         successClosure: @escaping ([String: Any]) -> Void,
         errorClosure: @escaping (SlackError) -> Void
     ) {
-        guard let url = self.url(for: endpoint, parameters: parameters) else {
+        guard let url = requestURL(for: endpoint, parameters: parameters) else {
             errorClosure(SlackError.clientNetworkError)
             return
         }

        }
    }

    private func url(for endpoint: Endpoint, parameters: [String: Any?]) -> URL? {
        var components = URLComponents(string: "\(apiUrl)\(endpoint.rawValue)")
        if parameters.count > 0 {
            components?.queryItems = filterNilParameters(parameters).map { URLQueryItem(name: $0.0, value: "\($0.1)") }

        successClosure: @escaping ([String: Any]) -> Void, errorClosure: @escaping (SlackError) -> Void
    ) {
        guard
            let url = requestURL(for: .filesUpload, parameters: parameters),
            let filename = parameters["filename"] as? String,
            let filetype = parameters["filetype"] as? String
        else {
