How to close a modalDialog from a webpart page in Sharepoint 2010

Hello all,

In sharepoint (2010 foundation) I am trying to use the SP.UI.ModalDialog.showModalDialog function to open a webpart page.
(and it succeeds!!)
However: closing the modalDialog fails. It seems I am closing the webpart page displayed inside de modalDialog but the dialog itself stays open.
The webpart used is a SmartPart, which reads the contents of an aspx file from the server.

This is the code I use to open the dialog:

<script language='text/javascript'>
function UurRegistratieWBS(LineNr, lblAfk, projectNr, BudgetCode) {
            
        var options = SP.UI.$create_DialogOptions();
        options.url = '/ERP/SitePages/UurRegistratie.aspx?wbsnr=' + LineNr + '&user=' + lblAfk + '&project=' + projectNr + '&BudgetCode=' + BudgetCode;
        options.title = "Tijdsregistratie WBS";
        options.showClose = true;
        options.autoSize = true;
        options.dialogReturnValueCallback = Function.createDelegate(null, CloseDialog);

         SP.UI.ModalDialog.showModalDialog(options);
      }
   </script>

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...And below is the code I (try to) use in the webpart page :
 

Protected Sub btnCancel_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnSluiten.Click

    Me.Response.Write("<script language='text/javascript'>SP.UI.ModalDialog.close(SP.UI.DialogResult.cancel);</script>")
    Me.Parent.Page.Response.Clear()
    Me.Parent.Page.Response.Write("<script language='text/javascript'>SP.UI.ModalDialog.close(SP.UI.DialogResult.cancel);</script>")
  End Sub

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As said earlier: the webpart page itself is closed, but the dialogbox stays open and has to be closed manually.
Can someone help me with this?

Thanks,

    Paul