Do you know why I always got this exception?

this is my code
private void webBrowser1_ProgressChanged(object sender, WebBrowserProgressChangedEventArgs e)
{
toolStripProgressBar1.Maximum = (int)e.MaximumProgress;
toolStripProgressBar1.Value = (int)e.CurrentProgress;
}
I would appreciate any answer, thank you.
According to documentation – property
CurrentProgressof event argument classWebBrowserProgressChangedEventArgshas value -1 when download is complete.Please use following code to ensure that progress bar completely reflects status of the download: