         failure: ((Error) -> Void)? = nil,
         success: @escaping (VoiceModels) -> Void)
     {
+        // construct header parameters
+        var headers = defaultHeaders
+        headers["Accept"] = "application/json"
+
         // construct query parameters
         var queryParameters = [URLQueryItem]()
         if let language = language {

            method: "GET",
            url: serviceURL + "/v1/pronunciation",
            credentials: credentials,
            headerParameters: defaultHeaders,
            acceptType: "application/json",
            queryItems: queryParameters,
            messageBody: nil
        )

        // execute REST request

            return
        }

        // construct header parameters
        var headers = defaultHeaders
        headers["Content-Type"] = "application/json"

        // construct REST request
        let path = "/v1/customizations/\(customizationID)/words"
        guard let encodedPath = path.addingPercentEncoding(withAllowedCharacters: .urlPathAllowed) else {
