need help in weblclient.download through an error like illegal character in path

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C#

HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://api.smartsheet.com/1.1/attachment/7951147966195588");
                request.Headers.Add("Authorization", "Bearer 672wbhnr4gt8rd5tvh9lw9n3o7");
                request.PreAuthenticate = true;
                request.Method = "GET";
                var response = (HttpWebResponse)request.GetResponse();
                Stream dataStream = response.GetResponseStream();
                StreamReader reader = new StreamReader(dataStream);
                string responseFromServer = reader.ReadToEnd();
                int i = responseFromServer.IndexOf(',')+1;
               string s = responseFromServer.Substring(i);
                int i1 = s.IndexOf(',')-6;
               string s1 = s.Substring(6,i1);
               WebClient wb = new WebClient();
               wb.Headers.Add("Authorization", "Bearer 672wbhnr4gt8rd5tvh9lw9n3o7");
               byte[] myDataBuffer = wb.DownloadData(s1);

ERROR:-
System.ArgumentException: Illegal characters in path. at System.IO.Path.CheckInvalidPathChars(String path) at System.IO.Path.NormalizePath(String path, Boolean fullCheck, Int32 maxPathLength) at System.IO.Path.GetFullPathInternal(String path) at System.IO.Path.GetFullPath(String path) at System.Net.WebClient.GetUri(String path) at System.Net.WebClient.DownloadData(String address) at WebApplication4.WebForm1.Button7_Click1(Object sender, EventArgs e) in D:\RMS\visualstudio\Projects\WebApplication4\WebApplication4\WebForm1.aspx.cs:line 210