WPF - InvalidOperationException On Second Time Executing BackgroundWorker

I'm making an app using wpf and vb.net in vb2010 express and having a problem. I have a timer that execute a backgroundworker every minute:
Private Sub timer_Tick() Handles timer.Tick
If Not bworker.IsBusy Then
bworker.RunWorkerAsync()
End If
End Sub

The backgroundworker is working with xmlelement and xmldataprovider:
Private Sub bworker_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles bworker.DoWork
Dim source As XmlDataProvider = Application.Current.Resources("r1")
Dim sometext As String = "something"
Dim elemento As XmlElement = source.Document.CreateElement("elemento")
elemento.InnerText = sometext
e.Result = elemento
End Sub

Then I'm using e.Result on RunWorkerComplete to add the XmlElement to source. It works perfect the first time I execute the backgroundworker, but when the timer calls it the second time it throws a InvalidOperationException in the line "elemento.InnerText = sometext" why is that and how can I solve it?