Error : The ServicePointManager does not support proxies of https scheme

Question

I need some help ,when I run the project I found Error :
" An unhandled exception of type 'System.NotSupportedException' occurred in system.dll
Additional information: The ServicePointManager does not support proxies of https scheme."
in Line : "System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)base.GetWebRequest(uri);"
in Code :

		protected override System.Net.WebRequest GetWebRequest(Uri uri)
		{
			System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)base.GetWebRequest(uri);
			if (this.PreAuthenticate)
			{
				System.Net.NetworkCredential nc = this.Credentials.GetCredential(uri,"Basic");
				if (nc != null)
				{
					byte[] credBuf = new System.Text.UTF8Encoding().GetBytes(nc.UserName + ":" + nc.Password);
					request.Headers["Authorization"] = "Basic " + Convert.ToBase64String(credBuf);
				}
			}

			return request;
		}

Please help me