Object synchronization method was called from an unsynchronized block of code.

Hi all Im downloading Pdfs using WebClient I used below code

try
                {
                    WebClient wc = new WebClient();
                    Uri uriadd = new Uri(@"ftp://xxx.yyy..../httpdocs/FH/Foldername/" + clientorder1[i].Cloi_id + ".pdf");
                    wc.Credentials = new NetworkCredential("xxxx", "zzzz");
}

its working fine  but some times Gives Error like Object synchronization method was called from an unsynchronized block of code.. Why this Error coming any solutions to Solve this.. How can Prevent this Error ....? any ideas..

thanks