Specified cast is not valid error

Hi Experts,

Can someone tell me why the 'Do Until' line is causing a 'Specified cast is not valid' error?

    Private Sub BackgroundWorker1_DoWork(sender As System.Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork

        WebBrowser1.Navigate("http://www.yahoo.com")

        Do Until WebBrowser1.ReadyState = WebBrowserReadyState.Complete
            Threading.Thread.Sleep(10)
            Application.DoEvents()
        Loop

    End Sub

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